PXR: Structure-specific activation by hepatotoxic pyrrolizidine alkaloids

Modeling HepaRG metabolome responses to pyrrolizidine alkaloid exposure for insight into points of departure and modes of action

The new challenges in toxicology demand novel and innovative in vitro approaches for deriving points of departure (PODs) and determining the mode of action (MOA) of chemicals. Therefore, the aim of this original study was to couple in vitro studies with untargeted metabolomics to model the concentration-response of extra- and intracellular metabolome data on human HepaRG cells treated for 48 h with three pyrrolizidine alkaloids (PAs): heliotrine, retrorsine and lasiocarpine. Modeling revealed that the three PAs induced various monotonic and, importantly, biphasic curves of metabolite content. Based on unannotated metabolites, the endometabolome was more sensitive than the exometabolome in terms of metabolomic effects, and benchmark concentrations (BMCs) confirmed that lasiocarpine was the most hepatotoxic PA. Regarding its MOA, impairment of lipid metabolism was highlighted at a very low BMC (first quartile, 0.003 µM). Moreover, results confirmed that lasiocarpine targets bile acids, as well as amino acid and steroid metabolisms. Analysis of the endometabolome, based on coupling concentration-response and PODs, gave encouraging results for ranking toxins according to their hepatotoxic effects. Therefore, this novel approach is a promising tool for next-generation risk assessment, readily applicable to a broad range of compounds and toxic endpoints.

PBTK model-based analysis of CYP3A4 induction and the toxicokinetics of the pyrrolizidine alkaloid retrorsine in man

Cytochrome P450 (CYP)3A4 induction by drugs and pesticides plays a critical role in the enhancement of pyrrolizidine alkaloid (PA) toxicity as it leads to increased formation of hepatotoxic dehydro-PA metabolites. Addressing the need for a quantitative analysis of this interaction, we developed a physiologically-based toxicokinetic (PBTK) model. Specifically, the model describes the impact of the well-characterized CYP3A4 inducer rifampicin on the kinetics of retrorsine, which is a prototypic PA and contaminant in herbal teas. Based on consumption data, the kinetics after daily intake of retrorsine were simulated with concomitant rifampicin treatment. Strongest impact on retrorsine kinetics (plasma AUC24 and C max reduced to 67% and 74% compared to the rifampicin-free reference) was predicted directly after withdrawal of rifampicin. At this time point, the competitive inhibitory effect of rifampicin stopped, while CYP3A4 induction was still near its maximum. Due to the impacted metabolism kinetics, the cumulative formation of intestinal retrorsine CYP3A4 metabolites increased to 254% (from 10 to 25 nmol), while the cumulative formation of hepatic CYP3A4 metabolites was not affected (57 nmol). Return to baseline PA toxicokinetics was predicted 14 days after stop of a 14-day rifampicin treatment. In conclusion, the PBTK model showed to be a promising tool to assess the dynamic interplay of enzyme induction and toxification pathways.

Environmental endocrine disruptors and pregnane X receptor action: A review

The pregnane X receptor (PXR) is a kind of orphan nuclear receptor activated by a series of ligands. Environmental endocrine disruptors (EEDs) are a wide class of molecules present in the environment that are suspected to have adverse effects on the endocrine system by interfering with the synthesis, transport, degradation, or action of endogenous hormones. Since EEDs may modulate human/rodent PXR, this review aims to summarize EEDs as PXR modulators, including agonists and antagonists. The modular structure of PXR is also described, interestingly, the pharmacology of PXR have been confirmed to vary among different species. Furthermore, PXR play a key role in the regulation of endocrine function. Endocrine disruption of EEDs via PXR and its related pathways are systematically summarized. In brief, this review may provide a way to understand the roles of EEDs in interaction with the nuclear receptors (such as PXR) and the related pathways.

Comparative Pharmacokinetic Study of Two Pyrrolizidine Alkaloids Lasiocarpine and Heliotrine in Rats

Lasiocarpine (LAS) and heliotrine (HEL) are two different ester types of toxic pyrrolizidine alkaloids (PAs): open-chain diester and monoester. However, the pharmacokinetics of these two types of PAs in rats have not been reported. In the present study, two LC-MS/MS methods for determining LAS and HEL were established and validated. The methods exhibited good linearity, accuracy, and precision and were then applied to a comparative pharmacokinetic study. After intravenous administration to male rats at 1 mg/kg, the AUC_0-t values of LAS and HEL were 336 ± 26 ng/mL × h and 170 ± 5 ng/mL × h. After oral administration at 10 mg/kg, the AUC_0-t of LAS was much lower than that of HEL (18.2 ± 3.8 ng/mL × h vs. 396 ± 18 ng/mL × h), while the C_max of LAS was lower than that of HEL (51.7 ± 22.5 ng/mL × h vs. 320 ± 26 ng/mL × h). The absolute oral bioavailability of LAS was 0.5%, which was significantly lower than that of HEL (23.3%). The results revealed that the pharmacokinetic behaviors of LAS differed from that of HEL.

Use of transcriptomics in hazard identification and next generation risk assessment: A case study with clothianidin

Toxicological risk assessment is essential in the evaluation and authorization of different classes of chemical substances. Genotoxicity and mutagenicity testing are of highest priority and rely on established in vitro systems with bacterial and mammalian cells, sometimes followed by in vivo testing using rodent animal models. Transcriptomic approaches have recently also shown their value to determine transcript signatures specific for genotoxicity. Here, we studied how transcriptomic data, in combination with in vitro tests with human cells, can be used for the identification of genotoxic properties of test compounds. To this end, we used liver samples from a 28-day oral toxicity study in rats with the pesticidal active substances imazalil, thiacloprid, and clothianidin, a neonicotinoid-type insecticide with, amongst others, known hepatotoxic properties. Transcriptomic results were bioinformatically evaluated and pointed towards a genotoxic potential of clothianidin. In vitro Comet and γH2AX assays in human HepaRG hepatoma cells, complemented by in silico analyses of mutagenicity, were conducted as follow-up experiments to check if the genotoxicity alert from the transcriptomic study is in line with results from a battery of guideline genotoxicity studies. Our results illustrate the combined use of toxicogenomics, classic toxicological data and new approach methods in risk assessment. By means of a weight-of-evidence decision, we conclude that clothianidin does most likely not pose genotoxic risks to humans.

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Analysis of clothianidin genotoxicity in silico, in vitro and in vivo.

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Application of a toxicogenomics approach to analyze genotoxicity.

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Weight-of-evidence decision supports classification as “non-genotoxic”.

Isolation of Echimidine and Its C-7 Isomers from Echium plantagineum L. and Their Hepatotoxic Effect on Rat Hepatocytes

Echimidine is the main pyrrolizidine alkaloid of Echium plantagineum L., a plant domesticated in many countries. Because of echimidine’s toxicity, this alkaloid has become a target of the European Food Safety Authority regulations, especially in regard to honey contamination. In this study, we determined by NMR spectroscopy that the main HPLC peak purified from zinc reduced plant extract with an MS [M + H]+ signal at m/z 398 corresponding to echimidine (1), and in fact also represents an isomeric echihumiline (2). A third isomer present in the smallest amount and barely resolved by HPLC from co-eluting (1) and (2) was identified as hydroxymyoscorpine (3). Before the zinc reduction, alkaloids (1) and (2) were present mostly (90%) in the form of an N-oxide, which formed a single peak in HPLC. This is the first report of finding echihumiline and hydroxymyoscorpine in E. plantagineum. Retroanalysis of our samples of E. plantagineum collected in New Zealand, Argentina and the USA confirmed similar co-occurrence of the three isomeric alkaloids. In rat hepatocyte primary culture cells, the alkaloids at 3 to 300 µg/mL caused concentration-dependent inhibition of hepatocyte viability with mean IC50 values ranging from 9.26 to 14.14 µg/mL. Our discovery revealed that under standard HPLC acidic conditions, echimidine co-elutes with its isomers, echihumiline and to a lesser degree with hydroxymyoscorpine, obscuring real alkaloidal composition, which may have implications for human toxicity.

Identification of microRNAs Implicated in Modulating Senecionine-Induced Liver Toxicity in HepaRG Cells

1,2-unsaturated Pyrrolizidine Alkaloids (PAs) are secondary plant metabolites that occur as food contaminants. Upon consumption, they can cause severe liver damage. PAs have been shown to induce apoptosis, to have cytotoxic and genotoxic effects, and to impair bile acid homeostasis in the human hepatoma cell line HepaRG. The major mode of action of PAs is DNA- and protein-adduct formation. Beyond that, nuclear receptor activation has only been observed for one receptor and two PAs, yielding the possibility that other cellular mediators are involved in PA-mediated toxicity. Here, the mode of action of Senecionine (Sc), a prominent and ubiquitous representative of hepatotoxic PAs, was investigated by analyzing 7 hepatic microRNAs (miRNAs) in HepaRG cells. Ultimately, 11 target genes that were predicted with Ingenuity Pathway Analysis software (IPA) were found to be significantly downregulated, while their assigned miRNAs showed significant upregulation of gene expression. According to IPA, these targets are positively correlated with apoptosis and cellular death and are involved in diseases such as hepatocellular carcinoma. Subsequent antagomiR-inhibition analysis revealed a significant correlation between PA-induced miRNA-4434 induction and P21-Activated Kinase-1 (PAK1) downregulation. PAK1 downregulation is usually associated with cell cycle arrest, suggesting a new function of Sc-mediated toxicity in human liver cells.

Organic Cation Transporter I and Na+ /taurocholate Co-Transporting Polypeptide are Involved in Retrorsine- and Senecionine-Induced Hepatotoxicity in HepaRG cells

SCOPE

1,2-unsaturated pyrrolizidine alkaloids (PAs) are secondary plant metabolites that are found in many plant species throughout the world. They are of concern for risk assessment as consumption of contaminated foodstuff can cause severe liver damage. Of late, transporter-mediated uptake and transport has advanced as a vital determinant of PA toxicity. In this study, the authors investigate a transporter-mediated uptake of PAs and its implications in PA toxicity.

METHODS AND RESULTS

We show that transporter expression levels are significantly affected by treatment with the PAs senecionine (Sc) and retrorsine (Re) in the human hepatoma cell line HepaRG. Furthermore, the specific contribution to PA uptake of the two transporters Na⁺ /taurocholate co-transporting polypeptide (SLC10A1) and organic cation transporter I (SLC22A1), both belonging to the heterogeneous solute carrier super family, is investigated by means of a siRNA-mediated knockdown approach. Knockdown of both uptake transporters result in reduced uptake of Re and Sc in a time-dependent manner and attenuated PA-mediated cytotoxic effects in HepaRG cells.

CONCLUSION

Our results confirm previous findings of active transport mechanisms of PAs into hepatocytes and highlight the importance of toxicokinetic studies for the risk assessment of PAs.

Prenatal Exposure to Retrorsine Induces Developmental Toxicity and Hepatotoxicity of Fetal Rats in a Sex-Dependent Manner: The Role of Pregnane X Receptor Activation

Pyrrolizidine alkaloids (PAs) are a type of natural phytotoxin that contaminate food and feed and become an environmental health risk to humans and livestock. PAs exert toxicity that requires metabolic activation by cytochrome P450 (CYP) 3A, and case reports showed that fetuses are quite susceptible to PAs toxicity. The aim of this study was to explore the characteristics of developmental toxicity and fetal hepatotoxicity induced by retrorsine (RTS, a typcial toxic PA) and the underlying mechanism. Pregnant Wistar rats were intragastrically administered with 20 mg/(kg·day) RTS from gestation day (GD) 9 to 20. Results showed that prenatal RTS exposure lowered fetal bodyweights, reduced hepatocyte numbers, and potentiated hepatic apoptosis in fetuses, particularly females. Simutaneously, RTS increased CYP3A expression and pregnane X receptor (PXR) activation in female fetal liver. We further confirmed that RTS was a PXR agonist in LO2 and HepG2 cell lines. Furthermore, agonism or antagonism of androgen receptor (AR) either induced or blocked RTS-mediated PXR activation, respectively. As a PXR agonist, RTS toxicity was exacerbated in female fetus due to the increased CYP3A induction and self-metabolism, while the inhibitory effect of AR on PXR activation reduced the susceptibility of male fetus to RTS. Our findings indicated that PXR may be a potential therapeutic target for PA toxicity.

Female-specific activation of pregnane X receptor mediates sex difference in fetal hepatotoxicity by prenatal monocrotaline exposure

Pyrrolizidine alkaloids (PAs) are a group of hepatic toxicant widely present in plants. Cytochrome P450 (CYP) 3A plays a key role in metabolic activation of PAs to generate electrophilic metabolites, which is the main cause of hepatotoxicity. We have previously demonstrated the sex difference in developmental toxicity and hepatotoxicity in fetal rats exposed to monocrotaline (MCT), a representative toxic PA. The aim of this study was to explore the underlying mechanism. 20 mg·kg^-1·d^-1 MCT was intragastrically given to pregnant Wistar rats from gestation day 9 to 20. CYP3As expression and pregnane X receptor (PXR) activation were specifically enhanced in female fetal liver. After MCT treatment, we also observed a significant increase of CYP3As expression in LO2 cells (high PXR level) or hPXR-transfected HepG2 cells (low PXR level). Employing hPXR and CYP3A4 dual-luciferase reporter gene assay, we confirmed the agonism effect of MCT on PXR-dependent transcriptional activity of CYP3A4. Agonism and antagonism of the androgen receptor (AR) either induced or blocked MCT-induced PXR activation, respectively. This study was the first report identifying that MCT served as PXR agonist to induce CYP3A expression. CYP3A induction may increase self-metabolic activation of MCT and subsequently lead to more severe hepatotoxicity in female fetus. While in male, during the intrauterine period, activated AR by testosterone secretion from developing testes represses MCT-induced PXR activation and CYP3A induction, which may partially protect male fetus from MCT-induced hepatotoxicity.

Alkaloids from Traditional Chinese Medicine against hepatocellular carcinoma

Hepatocellular carcinoma (HCC) has become one of the major diseases that are threatening human health in the 21st century. Currently there are many approaches to treat liver cancer, but each has its own advantages and disadvantages. Among various methods of treating liver cancer, natural medicine treatment has achieved promising results because of their superiorities of high efficiency and availability, as well as low side effects. Alkaloids, as a class of natural ingredients derived from traditional Chinese medicines, have previously been shown to exert prominent anti-hepatocarcinogenic effects, through various mechanisms including inhibition of proliferation, metastasis and angiogenesis, changing cell morphology, promoting apoptosis and autophagy, triggering cell cycle arrest, regulating various cancer-related genes as well as pathways and so on. As a consequence, alkaloids suppress the development and progression of liver cancer. In this study, the mechanisms of representative alkaloids against hepatocarcinoma in each class are described systematically according to the structure classification, which mainly divides alkaloids into piperidine alkaloids, isoquinoline alkaloids, indole alkaloids, terpenoids alkaloids, steroidal alkaloids and other alkaloids. Besides using them alone, synergistic effects created together with other chemotherapy drugs and some special preparation methods also have been demonstrated. In this review, we have summarized the potential roles of several common alkaloids in the prevention and treatment of HCC, by revising the preclinical studies, highlighting the potential applications of alkaloids when they function as a therapeutic choice for HCC treatment, and integrating them into clinical practices.

The influence of alkaloids on oxidative stress and related signaling pathways

Alkaloids have always attracted scientific interest due to either their positive or negative effects on human beings. This review aims to summarize their antioxidant effects by both classical in vitro scavenging assay and at the cellular level. Since most in vitro studies used the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging assay, the results from those studies are summed up in the first part of the article. In the second part, available data on the effect of alkaloids on NADPH-oxidase, the key enzyme for reactive oxygen species production, at the cellular level, are summarized. More than 130 alkaloids were tested by DPPH assay. However, due to methodological differences, a direct comparison is hardly possible. It can be at least concluded that some of them were either similar to or even more active than standard antioxidants and the number of aromatic hydroxyl groups seems to be the major determinant for the activity. The data on inhibition of NADPH-oxidase activity by alkaloids demonstrated that there is little relationship to the DPPH assay. The mechanism seems to be based on inhibition of synthesis, activation or translocation of NADPH-oxidase subunits. In some alkaloids, activation of the nuclear factor Nrf2 pathway was documented to be the grounds for inhibition of NADPH-oxidase. Interestingly, many alkaloids can behave both as anti-oxidants and pro-oxidants depending on conditions and pro-oxidation might be the reason for activation of Nrf2. Available data on other "antioxidant" transcription factors FOXOs and PPARs are also mentioned.

Sex difference in monocrotaline-induced developmental toxicity and fetal hepatotoxicity in rats

Pyrrolizidine alkaloids (PAs) are a class of hepatic toxins widely existing in plants. Cytochromes P450 (CYP) mediates PA bioactivation and toxicities in mammals. It has been reported that PAs can induce developmental toxicity, but systematic research is lacking. In this study, we investigated developmental toxicity of monocrotaline (MCT) in rats. Pregnant rats were administered with MCT (20 mg/kg) intragastrically from gestation day 9 to 20, followed by determination of changes in fetal growth, hepatic morphology, serum biochemical indices, and indicators of hepatocytes apoptosis. MCT was found to induce developmental toxicity and fetal hepatotoxicity, particularly in female fetuses. Metabolic activation was also studied by examination of bioactivation efficiency of MCT in fetal liver microsomes, serum MCT, pyrrole-protein adduction derived from MCT, and hepatic CYP3 A expression of fetuses in vivo. Male fetuses showed greater basal MCT bioactivation than that of female fetuses, but continuous exposure to MCT caused a selective CYP3 A induction in female fetuses, which may contribute to the sex difference in MCT-induced developmental toxicity.