Role of the nuclear receptor pregnane X receptor in acetaminophen hepatotoxicity

Forsythiaside A Reduces Acetaminophen Hepatotoxic Metabolism by Inhibiting Pregnane X Receptor

Overdose intake of acetaminophen (APAP) causes liver injury involving hepatic drug metabolism and activation of oxidative stress pathways, and forsythiaside A (FA) has hepatoprotective pharmacological activity, but knowledge of the mechanism of FA treatment for APAP liver injury is still lacking the literature. In this study, we investigated the effects of FA on the pregnane X receptor (PXR) by molecular docking and reporter gene assays. In addition, we explored the effects of FA on oxidative stress, endoplasmic reticulum stress (ERS), apoptosis, and hepatic pathology by interfering with PXR in ex vivo and in vivo models. The results showed that FA decreased the PXR protein expression level and effectively reduced the oxidative stress level in the APAP model. In addition, FA reduced the expression of ERS pathway ProteinkinaseR-likeERkinase (PERK)-translation initiation factor 2 (eIF-2α)-activating transcription factor 4 (ATF4) by inhibiting PXR, and at the same time, decreased the expression of apoptotic proteins C/EBP homologous protein (CHOP), Bax, Caspase 3, and Caspase 7, and elevated the expression of apoptosis-suppressing protein Bcl-2, which ultimately treated the hepatic pathology injury of APAP in mice. The present study confirmed that FA improved APAP metabolism by inhibiting PXR-mediated CYP1A2 and CYP3A11 and alleviated APAP-induced hepatic impairment by inhibiting hepatic oxidative stress, ERS, and apoptosis.

Post-COVID metabolic enzyme alterations in K18-hACE2 mice exacerbate alcohol-induced liver injury through transcriptional regulation

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), poses a significant threat to global public health. Despite reports of liver injury during viral disease, the occurrence and detailed mechanisms underlying the development of secondary exogenous liver injury, particularly in relation to changes in metabolic enzymes, remain to be fully elucidated. Therefore, this study was aimed to investigate the mechanisms underlying SARS-CoV-2-induced molecular alterations in hepatic metabolism and the consequent secondary liver injury resulting from alcohol exposure. We investigated the potential effects of SARS-CoV-2 infection on alcohol-induced liver injury in Keratin 18 promoter-human angiotensin converting enzyme 2 (K18-hACE2) transgenic mice. Mice were intranasally infected with 1 × 102 PFU of SARS-CoV-2. Following a 14 d recovery period from infection, the recovered mice were orally administered alcohol at 6 g/kg. Prior SARS-CoV-2 infection aggravated alcohol-induced liver injury based on increased alanine aminotransferase levels and cytoplasmic vacuolation. Interestingly, infected mice exhibited lower blood alcohol levels and higher levels of acetaldehyde, a toxic alcohol metabolite, compared to uninfected mice after the same period of alcohol consumption. Along with alterations of several metabolic process-related terms identified through RNA sequencing, notably, upregulation of cytochrome P450 2E1 (CYP2E1) and CYP1A2 was observed in infected mice compared to control value prior to alcohol exposure, with no significant impact of SARS-CoV-2 on intestinal damage. Tumor necrosis factor-alpha persistently showed upregulated expression in the infected mice; it also enhanced aryl hydrocarbon receptor and Sp1 expressions and their binding activity to Cyp1a2 and Cyp2e1 promoters, respectively, in hepatocytes, promoting the upregulation of their transcription. Our findings suggest that SARS-CoV-2 infection exacerbates alcohol-induced liver injury through the transcriptional activation of Cyp1a2 and Cyp2e1, providing valuable insights for the development of clinical recommendations on long COVID.

S-nitrosylation attenuates pregnane X receptor hyperactivity and acetaminophen-induced liver injury

Drug-induced liver injury (DILI), especially acetaminophen overdose, is the leading cause of acute liver failure. Pregnane X receptor (PXR) is a nuclear receptor and the master regulator of drug metabolism. Aberrant activation of PXR plays a pathogenic role in the acetaminophen hepatotoxicity. Here, we aimed to examine the S-nitrosylation of PXR (SNO-PXR) in response to acetaminophen. We found that PXR was S-nitrosylated in hepatocytes and the mouse livers after exposure to acetaminophen or S-nitrosoglutathione (GSNO). Mass spectrometry and site-directed mutagenesis identified the cysteine 307 as the primary residue for S-nitrosylation (SNO) modification. In hepatocytes, SNO suppressed both agonist-induced (rifampicin and SR12813) and constitutively active PXR (VP-PXR, a human PXR fused to the minimal transactivator domain of the herpes virus transcription factor VP16) activations. Furthermore, in acetaminophen-overdosed mouse livers, PXR protein was decreased at the centrilobular regions overlapping with increased SNO. In PXR-/- mice, replenishing the livers with the SNO-deficient PXR significantly aggravated hepatic necrosis, increased HMGB1 release, and exacerbated liver injury and inflammation. Particularly, we demonstrated that S-nitrosoglutathione reductase (GSNOR) inhibitor N6022 promoted hepatoprotection by increasing the levels of SNO-PXR. In conclusion, PXR is posttranslationally modified by SNO in hepatocytes in response to acetaminophen. This modification mitigated the acetaminophen-induced PXR hyperactivity. It may serve as a target for therapeutical intervention.

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.

Prevention of D-GalN/LPS-induced ALI by 18β-glycyrrhetinic acid through PXR-mediated inhibition of autophagy degradation

Acute liver injury (ALI) has multiple causes and results in liver dysfunction. Severe or persistent liver injury eventually leads to liver failure and even death. Pregnane X receptor (PXR)-null mice present more severe liver damage and lower rates of autophagy. 18β-glycyrrhetinic acid (GA) has been proposed as a promising hepatoprotective agent. We hypothesized that GA significantly alleivates D-GalN/LPS-induced ALI, which involved in PXR-mediated autophagy and lysosome biogenesis. We found that GA can significantly decrease hepatocyte apoptosis and increase the hepatic autophagy marker LC3-B. Ad-mCherry-GFP-LC3 tandem fluorescence, RNA-seq and real-time PCR indicated that GA may stabilize autophagosomes and lysosomes and inhibit autophagosome-lysosome fusion. Simultaneously, GA markedly activates PXR, even reversing the D-GalN/LPS-induced reduction of PXR and its downstream genes. In contrast, GA has a weak protective effect in pharmacological inhibition of PXR and PXR-null mice, which significantly affected apoptosis- and autophagy-related genes. PXR knockout interferes with the stability of autophagosomes and lysosomes, preventing GA reducing the expression of lysosomal genes such as Cst B and TPP1, and suppressing autophagy flow. Therefore, we believe that GA increases autophagy by inhibiting autophagosome-lysosome fusion and blocked autophagy flux via activation of PXR. In conclusion, our results show that GA activates PXR to regulate autophagy and lysosome biogenesis, represented by inhibiting autophagosome-lysosome fusion and stabilization of lysosome. These results identify a new mechanism by which GA-dependent PXR activation reduces D-GalN/LPS-induced acute liver injury.

Activation of Pregnane X Receptor-Cytochrome P450s Axis: A Possible Reason for the Enhanced Accelerated Blood Clearance Phenomenon of PEGylated Liposomes In Vivo

Recently, we reported that repeated injection of PEGylated liposomes (PEG-L) at certain intervals to the same rat lead to the disappearance of their long-circulation properties, referred to as the "accelerated blood clearance (ABC) phenomenon". Evidence from our recent studies suggested that cytochrome P450s (P450s) contribute to induction of the ABC phenomenon, a possibility that had been previously ignored. However, few details are known about the mechanism for induction of P450s. The present study was undertaken to investigate the roles in the ABC phenomenon of pregnane X receptor (PXR) and constitutive androstane receptor (CAR), the major upstream transcriptional regulators of the P450 genes, including CYP3A1, CYP2C6, and CYP1A2. The results demonstrated that expression of rat PXR and CAR was significantly increased in the ABC phenomenon and was accompanied by elevated CYP3A1, CYP2C6, and CYP1A2 levels. Further findings revealed that PXR but not CAR protein was substantially upregulated in the hepatocyte nucleus, together with marked nuclear colocalization of the PXR-retinoid X receptor alpha (RXRα) transcriptionally active heterodimer, indicating that nuclear translocation of PXR was induced in the ABC phenomenon, whereas nuclear translocation of CAR was not observed. Notably, pretreatment with the specific PXR inducer dexamethasone significantly induced accelerated systemic clearance of the subsequent injection of PEG-L, associating with increased nuclear colocalization of PXR-RXRα These results revealed that the induction of P450s in the ABC phenomenon may be attributable largely to the activation of PXR induced by sequential injections of PEG-L, thus confirming the crucial involvement of the PXR-P450s axis in promoting the ABC phenomenon. SIGNIFICANCE STATEMENT: The results of this study revealed that the induction of P450s in the ABC phenomenon may be largely attributable to the activation of PXR induced by sequential injections of PEG-L, thus confirming the crucial involvement of the PXR-P450s axis in promoting the ABC phenomenon. The data may help to extend our insights into 1) the role of P450s, which are regulated by the liver-enriched nuclear receptor PXR, in the ABC phenomenon, and 2) the therapeutic potential of targeting the PXR-P450 axis for reducing the magnitude of the ABC phenomenon in clinical practice.

Poly(ADP-ribosyl)ated PXR is a critical regulator of acetaminophen-induced hepatotoxicity

Acetaminophen (APAP) overdose is the most frequent cause of acute liver failure and remains a critical problem in medicine. PARP1-dependent poly(ADPribosyl)ation is a key mediator of cellular stress responses and functions in multiple physiological and pathological processes. However, whether it is involved in the process of APAP metabolism remains elusive. In this study, we find that PARP1 is activated in mouse livers after APAP overdose. Pharmacological or genetic manipulations of PARP1 are sufficient to suppress the APAP-induced hepatic toxicity and injury, as well as reduced APAP metabolism. Mechanistically, we identify pregnane X receptor (PXR) as a substrate of PARP1-mediated poly(ADP-ribosyl)ation. The poly(ADP-ribosyl)ation of PXR in ligand-binding domain activates PXR competitively and solidly, facilitates its recruitment to target gene CYP3A11 promoter, and promotes CYP3A11 gene transcription, thus resulting in increases of APAP pro-toxic metabolism. Additionally, PXR silence antagonizes the effects of PARP1 on APAP-induced hepatotoxicity. These results identifies poly(ADP-ribosyl)ation of PXR by PARP1 as a key step in APAP-induced liver injury. We propose that inhibition of PARP1-dependent poly(ADP-ribosyl)ation might represent a novel approach for the treatment of drug-induced hepatotoxicity.

Acetylation of lysine 109 modulates pregnane X receptor DNA binding and transcriptional activity

Pregnane X receptor (PXR) is a major transcriptional regulator of xenobiotic metabolism and transport pathways in the liver and intestines, which are critical for protecting organisms against potentially harmful xenobiotic and endobiotic compounds. Inadvertent activation of drug metabolism pathways through PXR is known to contribute to drug resistance, adverse drug-drug interactions, and drug toxicity in humans. In both humans and rodents, PXR has been implicated in non-alcoholic fatty liver disease, diabetes, obesity, inflammatory bowel disease, and cancer. Because of PXR's important functions, it has been a therapeutic target of interest for a long time. More recent mechanistic studies have shown that PXR is modulated by multiple PTMs. Herein we provide the first investigation of the role of acetylation in modulating PXR activity. Through LC-MS/MS analysis, we identified lysine 109 (K109) in the hinge as PXR's major acetylation site. Using various biochemical and cell-based assays, we show that PXR's acetylation status and transcriptional activity are modulated by E1A binding protein (p300) and sirtuin 1 (SIRT1). Based on analysis of acetylation site mutants, we found that acetylation at K109 represses PXR transcriptional activity. The mechanism involves loss of RXRα dimerization and reduced binding to cognate DNA response elements. This mechanism may represent a promising therapeutic target using modulators of PXR acetylation levels. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

Acetaminophen induces accumulation of functional rat CYP3A via polyubiquitination dysfunction

Acetaminophen (APAP) is extensively used as an analgesic and antipyretic drug. APAP is partly metabolized to N-acetyl-p-benzoquinone imine, a reactive metabolite, by cytochrome P450 (CYP) 1A2, 2E1 and 3A4. Some reports have indicated that CYP3A protein production and its metabolic activity are induced by APAP in rats in vivo. The CYP3A subfamily is believed to be transcriptionally regulated by chemical compounds. However, the mechanism underlying these responses is not completely understood. To clarify these mechanisms, we assessed the effects of APAP on CYP3A1/23 protein levels according to mRNA synthesis and protein degradation in rat hepatocyte spheroids, a model of liver tissue, in vivo. APAP induced CYP3A1/23 protein levels and metabolic activity. However, no change in CYP3A1/23 mRNA levels was observed. Moreover, APAP prolonged the half-life of CYP3A1/23 protein. CYP3A is known to be degraded via the ubiquitin-proteasome system. APAP significantly was found to decrease levels of polyubiquitinated CYP3A1/23 and glycoprotein 78, an E3 ligase of CYP3A1/23. These findings demonstrate that APAP induces accumulation of functional CYP3A protein via inhibition of protein degradation. Our findings may lead to the determination of novel drug-drug interactions with APAP.

Bioavailability of paracetamol with/without caffeine in Egyptian patients with hepatitis C virus

PURPOSE

This study investigates the involvement of liver dysfunction in the modulation of paracetamol pharmacokinetic profile in genotype-4 HCV patients treated with either paracetamol alone (Para) or in combination with caffeine (Para-Caf).

METHODS

Twenty healthy volunteers and 20 Child-Pugh B HCV patients, each divided into two equal subgroups, were examined, whose liver/kidney functions were correlated with their main clinical manifestation. After an overnight fasting, healthy and hepatic subjects received either a single dose of Para (1000 mg paracetamol) or Para-Caf (1000 mg paracetamol/130 mg caffeine). Two milliliters of saliva samples were collected prior to and at different time-intervals after drug administration and analyzed using HPLC.

RESULTS

There was a noticeable increase in the mean concentration time profile of salivary paracetamol concentrations in hepatic patients, with concomitant decrease in paracetamol clearance (CLT), along with induction in the primary pharmacokinetic (PK) parameters, C max, AUC(0-8 h) and AUC(0-∞) (by about 95, 82, and 64 %, respectively, after treatment with Para, and 98, 96, and 101 %, respectively, after treatment with Para-Caf), when compared with the corresponding parameters in healthy subjects. Additionally, the healthy subjects treated with Para-Caf exhibited bioinequivalent increase in C max, K a, and t 1/2 with decrease in T max when compared with the healthy individuals treated with Para alone. A similar pattern was recorded in hepatic patients after addition of caffeine to paracetamol, with even augmented significant increase in K a and t 1/2 (by 100 and 32 %, respectively).

CONCLUSIONS

Liver dysfunction modified the PK of paracetamol expressed as earlier effective paracetamol concentration, with obvious decrease in its clearance. Caffeine induced faster absorption (evidenced by shorter T max and higher K a) and prolonged t 1/2 of paracetamol, the effects that were more profound in hepatic patients. Further studies are needed to evaluate the influence of liver damage on paracetamol pharmacokinetics whenever repeated dosing is applied, to avoid possible drug accumulation.

Loss of 5-lipoxygenase activity protects mice against paracetamol-induced liver toxicity

BACKGROUND AND PURPOSE

Paracetamol (acetaminophen) is the most widely used over-the-counter analgesic and overdosing with paracetamol is the leading cause of hospital admission for acute liver failure. 5-Lipoxygenase (5-LO) catalyses arachidonic acid to form LTs, which lead to inflammation and oxidative stress. In this study, we examined whether deletion or pharmacological inhibition of 5-LO could protect mice against paracetamol-induced hepatic toxicity.

EXPERIMENTAL APPROACH

Both genetic deletion and pharmacological inhibition of 5-LO in C57BL/6J mice were used to study the role of this enzyme in paracetamol induced liver toxicity. Serum and tissue biochemistry, H&E staining, and real-time PCR were used to assess liver toxicity.

KEY RESULTS

Deletion or pharmacological inhibition of 5-LO in mice markedly ameliorated paracetamol-induced hepatic injury, as shown by decreased serum alanine transaminase and aspartate aminotransferase levels and hepatic centrilobular necrosis. The hepatoprotective effect of 5-LO inhibition was associated with induction of the antitoxic phase II conjugating enzyme, sulfotransferase2a1, suppression of the pro-toxic phase I CYP3A11 and reduction of the hepatic transporter MRP3. In 5-LO(-/-) mice, levels of GSH were increased, and oxidative stress decreased. In addition, PPAR α, a nuclear receptor that confers resistance to paracetamol toxicity, was activated in 5-LO(-/-) mice.

CONCLUSIONS AND IMPLICATIONS

The activity of 5-LO may play a critical role in paracetamol-induced hepatic toxicity by regulating paracetamol metabolism and oxidative stress.

Retinoid-xenobiotic interactions: the Ying and the Yang

The literature provides compelling evidence pointing to tight metabolic interactions between retinoids and xenobiotics. These are extensive and important for understanding xenobiotic actions in the body. Within the body, retinoids affect xenobiotic metabolism and actions and conversely, xenobiotics affect retinoid metabolism and actions. This article summarizes data that establish the importance of retinoid-dependent metabolic pathways for sustaining the body's responses to xenobiotic exposure, including the roles of all-trans- and 9-cis-retinoic acid for protecting mammals from harmful xenobiotic effects and for ensuring xenobiotic elimination from the body. This review will also consider molecular mechanisms underlying xenobiotic toxicity focusing on how this may contribute to retinoid deficiency and disruption of normal retinoid homeostasis. Special attention is paid to xenobiotic molecular targets (nuclear receptors, regulatory proteins, enzymes, and transporters) which affect retinoid metabolism and signaling.

Pregnane X receptor and drug-induced liver injury

INTRODUCTION

The liver plays a central role in transforming and clearing foreign substances. The continuous exposure of the liver to xenobiotics sometimes leads to impaired liver function, referred to as drug-induced liver injury (DILI). The pregnane X receptor (PXR) tightly regulates the expression of genes in the hepatic drug-clearance system and its undesired activation plays a role in DILI.

AREAS COVERED

This review focuses on the recent progress in understanding PXR-mediated DILI and highlights the efforts made to assess and manage PXR-mediated DILI during drug development.

EXPERT OPINION

Future efforts are needed to further elucidate the mechanisms of PXR-mediated liver injury, including the epigenetic regulation and polymorphisms of PXR. Novel in vitro models containing functional PXR could improve our ability to predict and assess DILI during drug development. PXR inhibitors may provide chemical tools to validate the potential of PXR as a therapeutic target and to develop drugs to be used in the clinic to manage PXR-mediated DILI.

Targeting the pregnane X receptor in liver injury

INTRODUCTION

The nuclear receptor pregnane X receptor (PXR) is a well-characterized hepatic xenobiotic sensor whose activation by chemically diverse compounds results in the induction of drug clearance pathways that rid the body of potentially toxic substances, thus conferring protection from foreign chemicals and endobiotics.

AREAS COVERED

PXR activities are implicated in drug-drug interactions and endocrine disruption. Recent evidence supports a hepatoprotective role for PXR in chronic liver injury, inhibiting liver inflammation through suppression of the NF-κB pathway. However, PXR-mediated induction of CYP3A enhances APAP-induced acute liver injury by generating toxic metabolites. While these observations implicate PXR as a therapeutic target for liver injury, they also caution against PXR activation by pharmaceutical drugs.

EXPERT OPINION

While evidence of PXR involvement in acute and chronic liver injuries identifies it as a possible therapeutic target, it raises additional concerns for all drug candidates. The in vitro and in vivo tests for human PXR activation should be incorporated into the FDA regulations for therapeutic drug approval to identify potential liver toxicities. In addition, PXR pharmacogenetic studies will facilitate the prediction of patient-specific drug reactivities and associated liver disorders.

In vivo and in vitro characterization of a first-in-class novel azole analog that targets pregnane X receptor activation

The pregnane X receptor (PXR) is a master regulator of xenobiotic clearance and is implicated in deleterious drug interactions (e.g., acetaminophen hepatotoxicity) and cancer drug resistance. However, small-molecule targeting of this receptor has been difficult; to date, directed synthesis of a relatively specific PXR inhibitor has remained elusive. Here we report the development and characterization of a first-in-class novel azole analog [1-(4-(4-(((2R,4S)-2-(2,4-difluorophenyl)-2-methyl-1,3-dioxolan-4-yl)methoxy)phenyl)piperazin-1-yl)ethanone (FLB-12)] that antagonizes the activated state of PXR with limited effects on other related nuclear receptors (i.e., liver X receptor, farnesoid X receptor, estrogen receptor α, peroxisome proliferator-activated receptor γ, and mouse constitutive androstane receptor). We investigated the toxicity and PXR antagonist effect of FLB-12 in vivo. Compared with ketoconazole, a prototypical PXR antagonist, FLB-12 is significantly less toxic to hepatocytes. FLB-12 significantly inhibits the PXR-activated loss of righting reflex to 2,2,2-tribromoethanol (Avertin) in vivo, abrogates PXR-mediated resistance to 7-ethyl-10-hydroxycamptothecin (SN-38) in colon cancer cells in vitro, and attenuates PXR-mediated acetaminophen hepatotoxicity in vivo. Thus, relatively selective targeting of PXR by antagonists is feasible and warrants further investigation. This class of agents is suitable for development as chemical probes of PXR function as well as potential PXR-directed therapeutics.

A risk-benefit assessment of paracetamol (acetaminophen) combined with caffeine

OBJECTIVE

To determine the risk: benefit of paracetamol combined with caffeine in the short-term management of acute pain conditions.

DESIGN

Database searches were conducted to identify double-blind trials comparing paracetamol/caffeine with paracetamol alone (benefit analysis) and any data pertaining to hepatotoxicity of paracetamol when combined with caffeine (risk analysis).

INTERVENTIONS

Paracetamol/caffeine (1,000 mg/130 mg) vs paracetamol (1,000 mg) alone.

OUTCOME MEASURES

Assessment of benefit has been derived by meta-analysis. Information on the pain condition and number of patients studied, dosing regimen, study design and analgesic outcome measures (total pain relief scores) was extracted and dichotomous outcomes were obtained by calculating the number of patients in each treatment group who achieved at least 50% of the maximum total pain relief score. Assessment of risk has been made by appraisal of the literature.

RESULTS

Eight studies from four papers provided sufficient quantitative data for satisfactory meta-analysis. The relative benefit (of achieving at least 50% pain relief) of paracetamol/caffeine vs paracetamol alone was 1.12 (95% Confidence Interval 1.05-1.19) across a number of acute pain states (dysmenorrhoea, headache, post-partum pain, and dental pain). Review of the effects of the combination of paracetamol and caffeine on the liver revealed no compelling data to suggest a clinically meaningful increase in hepatotoxicity with use of paracetamol/caffeine combinations.

CONCLUSIONS

Paracetamol/caffeine (1,000 mg/130 mg) is effective and safe for use in acute management of pain. The hepatotoxicity of overdoses of paracetamol results from its oxidative metabolism, caffeine does not produce any increase in oxidative metabolism of therapeutic concentrations of paracetamol.

Orphan nuclear receptors as targets for drug development

Orphan nuclear receptors regulate diverse biological processes. These important molecules are ligand-activated transcription factors that act as natural sensors for a wide range of steroid hormones and xenobiotic ligands. Because of their importance in regulating various novel signaling pathways, recent research has focused on identifying xenobiotics targeting these receptors for the treatment of multiple human diseases. In this review, we will highlight these receptors in several physiologic and pathophysiologic actions and demonstrate how their functions can be exploited for the successful development of newer drugs.

Activation of the farnesoid X receptor provides protection against acetaminophen-induced hepatic toxicity

The nuclear receptor, farnesoid X receptor (FXR, NR1H4), is known to regulate cholesterol, bile acid, lipoprotein, and glucose metabolism. In the current study, we provide evidence to support a role for FXR in hepatoprotection from acetaminophen (APAP)-induced toxicity. Pharmacological activation of FXR induces the expression of several genes involved in phase II and phase III xenobiotic metabolism in wild-type, but not Fxr(-/-) mice. We used chromatin immunoprecipitation-based genome-wide response element analyses coupled with luciferase reporter assays to identify functional FXR response elements within promoters, introns, or intragenic regions of these genes. Consistent with the observed transcriptional changes, FXR gene dosage is positively correlated with the degree of protection from APAP-induced hepatotoxicity in vivo. Further, we demonstrate that pretreatment of wild-type mice with an FXR-specific agonist provides significant protection from APAP-induced hepatotoxicity. Based on these findings, we propose that FXR plays a role in hepatic xenobiotic metabolism and, when activated, provides hepatoprotection against toxins such as APAP.

Rifampicin-activated human pregnane X receptor and CYP3A4 induction enhance acetaminophen-induced toxicity

Acetaminophen (APAP) is safe at therapeutic levels but causes hepatotoxicity via N-acetyl-p-benzoquinone imine-induced oxidative stress upon overdose. To determine the effect of human (h) pregnane X receptor (PXR) activation and CYP3A4 induction on APAP-induced hepatotoxicity, mice humanized for PXR and CYP3A4 (TgCYP3A4/hPXR) were treated with APAP and rifampicin. Human PXR activation and CYP3A4 induction enhanced APAP-induced hepatotoxicity as revealed by hepatic alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities elevated in serum, and hepatic necrosis after coadministration of rifampicin and APAP, compared with APAP administration alone. In contrast, hPXR mice, wild-type mice, and Pxr-null mice exhibited significantly lower ALT/AST levels compared with TgCYP3A4/hPXR mice after APAP administration. Toxicity was coincident with depletion of hepatic glutathione and increased production of hydrogen peroxide, suggesting increased oxidative stress upon hPXR activation. Moreover, mRNA analysis demonstrated that CYP3A4 and other PXR target genes were significantly induced by rifampicin treatment. Urinary metabolomic analysis indicated that cysteine-APAP and its metabolite S-(5-acetylamino-2-hydroxyphenyl)mercaptopyruvic acid were the major contributors to the toxic phenotype. Quantification of plasma APAP metabolites indicated that the APAP dimer formed coincident with increased oxidative stress. In addition, serum metabolomics revealed reduction of lysophosphatidylcholine in the APAP-treated groups. These findings demonstrated that human PXR is involved in regulation of APAP-induced toxicity through CYP3A4-mediated hepatic metabolism of APAP in the presence of PXR ligands.

Elucidating the 'Jekyll and Hyde' nature of PXR: the case for discovering antagonists or allosteric antagonists

The pregnane X receptor belongs to the nuclear hormone receptor superfamily and is involved in the transcriptional control of numerous genes. It was originally thought that it was a xenobiotic sensor controlling detoxification pathways. Recent studies have shown an increasingly important role in inflammation and cancer, supporting its function in abrogating tissue damage. PXR orthologs and PXR-like pathways have been identified in several non-mammalian species which corroborate a conserved role for PXR in cellular detoxification. In summary, PXR has a multiplicity of roles in vivo and is being revealed as behaving like a "Jekyll and Hyde" nuclear hormone receptor. The importance of this review is to elucidate the need for discovery of antagonists of PXR to further probe its biology and therapeutic applications. Although several PXR agonists are already reported, virtually nothing is known about PXR antagonists. Here, we propose the development of PXR antagonists through chemical, genetic and molecular modeling approaches. Based on this review it will be clear that antagonists of PXR and PXR-like pathways will have widespread utility in PXR biology and therapeutics.

Alterations of chemotherapeutic pharmacokinetic profiles by drug-drug interactions

BACKGROUND

Drug interactions in oncology are common place and largely ignored as we tolerate high thresholds of 'toxic' drug responses in these patients. However, in the era of 'targeted' or seemingly 'less toxic' therapy, these interactions are more commonly flagged and contribute significantly towards poor 'quality of life' and medical fatalities.

OBJECTIVE

This review and opinion article focuses on alteration of chemotherapeutic pharmacokinetic profiles by drug interactions in the setting of polypharmacy. The assumption is that the drugs, with changes in their pharmacokinetics, will contribute towards changes in their pharmacodynamics.

METHODS

The examples cited for such drug-drug interactions are culled from published literature with an emphasis on those interactions that have been well characterized at the molecular level.

RESULTS

Although very few drug interaction studies have been performed on approved oncology based drugs, it is clear that drugs whose pharmacokinetics profiles are closely related to their pharmacodynamics will indeed result in clinically important drug interactions. Some newer mechanisms are described that involve interactions at the level of gene transcription, whereby, drug metabolism is significantly altered. However, for any given drug interaction, there does not seem to be a comprehensive model describing interactions.

CONCLUSIONS

Mechanisms based drug interactions are plentiful in oncology; however, there is an absolute lack of a comprehensive model that would predict drug-drug interactions.

Role of CYP3A and CYP2E1 in alcohol-mediated increases in acetaminophen hepatotoxicity: comparison of wild-type and Cyp2e1(-/-) mice

CYP2E1 is widely accepted as the sole form of cytochrome P450 responsible for alcohol-mediated increases in acetaminophen (APAP) hepatotoxicity. However, we previously found that alcohol [ethanol and isopentanol (EIP)] causes increases in APAP hepatotoxicity in Cyp2e1(-/-) mice, indicating that CYP2E1 is not essential. Here, using wild-type and Cyp2e1(-/-) mice, we investigated the relative roles of CYP2E1 and CYP3A in EIP-mediated increases in APAP hepatotoxicity. We found that EIP-mediated increases in APAP hepatotoxicity occurred at lower APAP doses in wild-type mice (300 mg/kg) than in Cyp2e1(-/-) mice (600 mg/kg). Although this result suggests that CYP2E1 has a role in the different susceptibilities of these mouse lines, our findings that EIP-mediated increases in CYP3A activities were greater in wild-type mice compared with Cyp2e1(-/-) mice raises the possibility that differential increases in CYP3A may also contribute to the greater APAP sensitivity in EIP-pretreated wild-type mice. At the time of APAP administration, which followed an 11 h withdrawal from the alcohols, alcohol-induced levels of CYP3A were sustained in both mouse lines, whereas CYP2E1 was decreased to constitutive levels in wild-type mice. The CYP3A inhibitor triacetyloleandomycin (TAO) decreased APAP hepatotoxicity in EIP-pretreated wild-type and Cyp2e1(-/-) mice. TAO treatment in vivo resulted in inhibition of microsomal CYP3A-catalyzed activity, measured in vitro, with no inhibition of CYP1A2 and CYP2E1 activities. In conclusion, these findings suggest that both CYP3A and CYP2E1 contribute to APAP hepatotoxicity in alcohol-treated mice.

Dietary inulin alleviates hepatic steatosis and xenobiotics-induced liver injury in rats fed a high-fat and high-sucrose diet: association with the suppression of hepatic cytochrome P450 and hepatocyte nuclear factor 4alpha expression

Inulin enzymatically synthesized from sucrose is a dietary component that completely escapes glucide digestion. Supplementing inulin to a high-fat and high-sucrose diet (HF) ameliorated hypertriglycemia and hepatic steatosis in 8-week-fed rats by suppressing elevated levels of serum triacylglycerols, fatty acids, and glucose, and the accumulation of hepatic triacylglycerols and fatty acids. Inulin intake prevented phenobarbital (PB)- and dexamethasone-induced liver injuries in the HF group. No significant alteration in the baseline expression of CYP2B, CYP2C11, CYP3A, and NADPH-cytochrome P450 (P450) reductase mRNAs and proteins was found. In contrast, baseline and PB-treated expressions of CYP2E1 mRNA were reduced in HF-fed rats. The induction of P450s in response to PB was affected by the nutritional status of the rats; mRNA levels of CYP2B1 and CYP3A1 after PB treatment, as assessed by quantitative real-time polymerase chain reaction analysis were reduced in the inulin-supplemented HF (HF+I) group, compared with those in the HF group. Western blot analysis detected the corresponding changes of CYP2B and CYP3A proteins. These alterations were correlated with changes in hepatic thiobarbituric acid-reactive substances. Furthermore, no significant difference in the expression of nuclear receptors constitutive androstane receptor, pregnane X receptor, and retinoid X receptor alpha and coactivator peroxisome proliferator-activated receptor-gamma coactivator 1alpha proteins was found in the hepatic nucleus between the HF and HF+I groups, but the expression of hepatocyte nuclear factor alpha (HNF4alpha) protein was significantly reduced in the HF+I group. Taken together, these results indicate that inulin intake ameliorates PB-induced liver injury, associated with a decline in lipid accumulation and PB-induced expression of CYP2B and CYP3A, which may be related by a reduction in the nuclear expression of HNF4alpha.

Mutual repression between steroid and xenobiotic receptor and NF-kappaB signaling pathways links xenobiotic metabolism and inflammation

While it has long been known that inflammation and infection reduce expression of hepatic cytochrome P450 (CYP) genes involved in xenobiotic metabolism and that exposure to xenobiotic chemicals can impair immune function, the molecular mechanisms underlying both of these phenomena have remained largely unknown. Here we show that activation of the nuclear steroid and xenobiotic receptor (SXR) by commonly used drugs in humans inhibits the activity of NF-kappaB, a key regulator of inflammation and the immune response. NF-kappaB target genes are upregulated and small bowel inflammation is significantly increased in mice lacking the SXR ortholog pregnane X receptor (PXR), thereby demonstrating a direct link between SXR and drug-mediated antagonism of NF-kappaB. Interestingly, NF-kappaB activation reciprocally inhibits SXR and its target genes whereas inhibition of NF-kappaB enhances SXR activity. This SXR/PXR-NF-kappaB axis provides a molecular explanation for the suppression of hepatic CYP mRNAs by inflammatory stimuli as well as the immunosuppressant effects of xenobiotics and SXR-responsive drugs. This mechanistic relationship has clinical consequences for individuals undergoing therapeutic exposure to the wide variety of drugs that are also SXR agonists.