Pyrrolizidine Alkaloids: Potential Role in the Etiology of Cancers, Pulmonary Hypertension, Congenital Anomalies, and Liver Disease

Micronucleus assays with the human derived liver cell line (Huh6): A promising approach to reduce the use of laboratory animals in genetic toxicology

The inadequate representation of enzymes which catalyze the activation/detoxification of xenobiotics in cells that are currently used in genotoxicity testing of chemicals leads to a high number of false positive results and the number of follow up studies with rodents could be reduced by use of more reliable in vitro models. We found earlier that several xenobiotic drug metabolizing enzymes are represented in the human derived liver cell line Huh6 and developed a protocol for micronucleus (MN) experiments which is in agreement with the current OECD guideline. This protocol was used to test 23 genotoxic and non-genotoxic reference chemicals; based on these results and of earlier findings (with 9 chemicals) we calculated the predictive value of the assay for the detection of genotoxic carcinogens. We found a sensitivity of 80% and a specificity of 94% for a total number of 32 chemicals; comparisons with results obtained with other in vitro assays show that the validity of MN tests with Huh6 is higher as that of other experimental models. These results are promising and indicate that the use of Huh6 cells in genetic toxicology may contribute to the reduction of the use of laboratory rodents; further experimental work to confirm this assumption is warranted.

CHOP Regulates Endoplasmic Reticulum Stress-Mediated Hepatoxicity Induced by Monocrotaline

Monocrotaline (MCT), a pyrrolizidine alkaloid, is the major toxin in Crotalaria, which causes cell apoptosis in humans and animals. It has been reported that the liver is a vulnerable target of MCT. However, the exact molecular mechanism of the interaction between endoplasmic reticulum (ER) stress and liver injury induced by MCT is still unclear. In this study, the cytotoxicity of MCT on primary rat hepatocytes was analyzed by a CCK-8 assay and Annexin V-FITC/PI assay. Protein expression was detected by western blotting and immunofluorescence staining. As a result, MCT significantly decreased the cell viability and mediated the apoptosis of primary rat hepatocytes. Meanwhile, MCT could also induce ER stress in hepatocytes, indicated by the expression of ER stress-related proteins, including GRP78, p-IRE1α, ATF6, p-eIF2α, ATF4, and CHOP. Pretreatment with 4-PBA, an inhibitor of ER stress, or knockdown of CHOP by siRNA could partly enhance cell viability and relieve the apoptosis. Our findings indicate that ER stress is involved in the hepatotoxicity induced by MCT, and CHOP plays an important role in this process.

Metabolism-mediated cytotoxicity and genotoxicity of pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) and PA N-oxides are common phytotoxins produced by over 6000 plant species. Humans are frequently exposed to PAs via ingestion of PA-containing herbal products or PA-contaminated foods. PAs require metabolic activation to form pyrrole-protein adducts and pyrrole-DNA adducts which lead to cytotoxicity and genotoxicity. Individual PAs differ in their metabolic activation patterns, which may cause significant difference in toxic potency of different PAs. This review discusses the current knowledge and recent advances of metabolic pathways of different PAs, especially the metabolic activation and metabolism-mediated cytotoxicity and genotoxicity, and the risk evaluation methods of PA exposure. In addition, this review provides perspectives of precision toxicity assessment strategies and biomarker development for the risk control and translational investigations of human intoxication by PAs.

Hepatic sinusoidal obstruction syndrome caused by the ingestion of Gynura segetum in a patient with alcoholic cirrhosis: a case report

Hepatic sinusoidal obstruction syndrome (HSOS) is a rare hepatic vascular disorder characterized by intrahepatic congestion, liver injury, and post-sinusoidal portal hypertension, and it is frequently associated with hematopoietic stem cell transplantation. In this study, we observed a case of HSOS associated with the ingestion of Gynura segetum, a pyrrolizidine alkaloid (PA)-containing Chinese herb, in a patient with alcoholic cirrhosis. The patient was a 43-year-old man with chief complaints of physical asthenia and a loss of appetite for more than a month. The diagnosis of HSOS combined with alcoholic cirrhosis was confirmed via the histopathological examination of liver tissues. With proper supportive and symptomatic care and anticoagulation therapy using low-molecular-weight heparin, the patient’s condition was stabilized. Because of its nonspecific symptoms in the early stage and a lack of information about PA consumption, PA-induced HSOS (PA-HSOS) has been long neglected, especially in patients with underlying liver diseases. Early identification and intervention are critical for optimizing outcomes. Further efforts are needed to supervise the use of PA-containing herbal medicines and identify accurate biomarkers for PA-HSOS.

LC-MS/MS method for determination of seneciphylline and its metabolite, seneciphylline N-oxide in rat plasma, and its application to a rat pharmacokinetic study

A rapid and sensitive ultra-performance liquid chromatography-tandem electrospray ionization mass spectrometry (UPLC-ESI/MS) method was established and validated for simultaneous determination of seneciphylline and its main metabolite in rat plasma. The plasma sample was prepared by simple methanol-mediated precipitation. Chromatographic separation was achieved within 3 min by gradient elution using acetonitrile and water containing 0.1% formic acid as mobile phase on a Waters ACQUITY BEH C18 column (100 × 2.1 mm, i.d. 1.7  μm). Quantitation was conducted in a positive multiple reaction monitoring mode. The linearity of the method was over the range of 1-1,000 ng/mL, with the lower limit of quantification of 1 ng/mL. The intra- and inter-day precision and accuracy, extraction recovery, and matrix effect of analytes were within the acceptable limit. The analytes were stable during the process of sample collection, preparation, and analysis. The validated method was further applied to a pharmacokinetic study of seneciphylline in rats after oral and intravenous administration. The results revealed that seneciphylline was quickly absorbed into plasma (T_max , 0.23-0.32 h) and reached the maximum concentration of 0.82-1.75 μg/mL after oral administration. Both seneciphylline and seneciphylline N-oxide were eliminated from plasma quickly. The low system exposure (oral bioavailability, 5.43-10.31%) was related to the extensive metabolism in the liver and microflora.

Pyrrolizidine alkaloids cause cell cycle and DNA damage repair defects as analyzed by transcriptomics in cytochrome P450 3A4-overexpressing HepG2 clone 9 cells

Pyrrolizidine alkaloids (PAs) are a large group of highly toxic chemical compounds, which are found as cross-contaminants in numerous food products (e.g., honey), dietary supplements, herbal teas, and pharmaceutical herbal medicines. PA contaminations are responsible for serious hepatotoxicity and hepatocarcinogenesis. Health authorities have to set legal limit values to guarantee the safe consumption of plant-based nutritional and medical products without harmful health. Toxicological and chemical analytical methods are conventionally applied to determine legally permitted limit values for PAs. In the present investigation, we applied a highly sensitive transcriptomic approach to investigate the effect of low concentrations of five PAs (lasiocarpine, riddelliine, lycopsamine, echimidine, and monocrotaline) on human cytochrome P450 3A4-overexpressing HepG2 clone 9 hepatocytes. The transcriptomic profiling of deregulated gene expression indicated that the PAs disrupted important signaling pathways related to cell cycle regulation and DNA damage repair in the transfected hepatocytes, which may explain the carcinogenic PA effects. As PAs affected the expression of genes that involved in cell cycle regulation, we applied flow cytometric cell cycle analyses to verify the transcriptomic data. Interestingly, PA treatment led to an arrest in the S phase of the cell cycle, and this effect was more pronounced with more toxic PAs (i.e., lasiocarpine and riddelliine) than with the less toxic monocrotaline. Using immunofluorescence, high fractions of cells were detected with chromosome congression defects upon PA treatment, indicating mitotic failure. In conclusion, the tested PAs revealed threshold concentrations, above which crucial signaling pathways were deregulated resulting in cell damage and carcinogenesis. Cell cycle arrest and DNA damage repair point to the mutagenicity of PAs. The disturbance of chromosome congression is a novel mechanism of Pas, which may also contribute to PA-mediated carcinogenesis. Transcriptomic, cell cycle, and immunofluorescence analyses should supplement the standard techniques in toxicology to unravel the biological effects of PA exposure in liver cells as the primary target during metabolization of PAs.

Use of Herbarium Voucher Specimens To Investigate Phytochemical Composition in Poisonous Plant Research

Poisonous plants cause large losses to the livestock industry through death, reduced production efficiency, reproductive dysfunction, and compromised harvesting of rangeland and pasture forages. Research investigating poisonous plants is complex because there are hundreds of genera of toxic plants representing thousands of species. To investigate the effects of poisonous plants on livestock, a clear understanding of the taxonomic identity of the plant and the ability to collect the plant in sufficient quantities for scientific studies is required. Subsequently, the active principles must be defined and investigated in the taxa of interest to better predict risk and make recommendations to reduce losses. Herbaria are collections of preserved plant specimens and are an important resource in poisonous plant research. Voucher specimens have often been used in the identification of the plant for the experimental reproduction of suspected livestock poisoning associated with a spontaneous case. More recently, herbarium specimens have been used to investigate the chemical composition of toxic plants as well as the distribution of different chemotypes over the landscape. The primary purpose of this review is to highlight the chemical analysis of herbarium specimens in poisonous plant research.

Active Transport of Hepatotoxic Pyrrolizidine Alkaloids in HepaRG Cells

1,2-unsaturated pyrrolizidine alkaloids (PAs) are secondary plant metabolites occurring as food contaminants that can cause severe liver damage upon metabolic activation in hepatocytes. However, it is yet unknown how these contaminants enter the cells. The role of hepatic transporters is only at the beginning of being recognized as a key determinant of PA toxicity. Therefore, this study concentrated on assessing the general mode of action of PA transport in the human hepatoma cell line HepaRG using seven structurally different PAs. Furthermore, several hepatic uptake and efflux transporters were targeted with pharmacological inhibitors to identify their role in the uptake of the PAs retrorsine and senecionine and in the disposition of their N-oxides (PANO). For this purpose, PA and PANO content was measured in the supernatant using LC-MS/MS. Also, PA-mediated cytotoxicity was analyzed after transport inhibition. It was found that PAs are taken up into HepaRG cells in a predominantly active and structure-dependent manner. This pattern correlates with other experimental endpoints such as cytotoxicity. Pharmacological inhibition of the influx transporters Na⁺/taurocholate co-transporting polypeptide (SLC10A1) and organic cation transporter 1 (SLC22A1) led to a reduced uptake of retrorsine and senecionine into HepaRG cells, emphasizing the relevance of these transporters for PA toxicokinetics.

Metabolic Pattern of Hepatotoxic Pyrrolizidine Alkaloids in Liver Cells

Contamination with 1,2-unsaturated pyrrolizidine alkaloids (PAs) is a serious problem for certain phytomedicines, foods, and animal feeds. Several of these PAs are genotoxic and carcinogenic, primarily in the liver, upon cytochrome P450 (CYP)-catalyzed activation into reactive (pyrrolic and pyrrole-like) metabolites. Here we investigated the metabolism of selected PAs (echimidine, europine, lasiocarpine, lycopsamine, retrorsine, and senecionine) in rat hepatocytes in primary culture and in human CYP3A4-transfected HepG2 cells. The open-chained diesters echimidine and lasiocarpine and the cyclic diester senecionine were extensively metabolized in rat hepatocytes into a broad spectrum of products released into the medium. A large portion of unidentified, possibly irreversibly bound, products remained in the cells while detectable amounts of reactive and other metabolites were found in the incubation media. In HepG2-CYP3A4 cells, lasiocarpine was more extensively metabolized than echimidine and senecionine which also gave rise to the release of pyrrolic metabolites. In human cells, no pyrrolic metabolites were detected in retrorsine or lycopsamine incubations, while no such metabolites were detected from europine in both cell types. Other types of metabolic changes comprised modifications such as side chain demethylation or oxygenation reactions like the formation of N-oxides. The latter, considered as a detoxification step, was a major pathway with cyclic diesters, was less distinctive for echimidine and lycopsamine and almost negligible for lasiocarpine and europine. Our data are in agreement with previously published cyto- and genotoxicity findings and suggests that the metabolic pattern may contribute substantially to the specific toxic potency of a certain congener. In addition, marked differences were found for certain congeners between rat hepatocytes and transfected human HepG2 cells, whereby a high level of bioactivation was found for lasiocarpine, whereas a very low level of bioactivation was observed for monoesters, in particular in human cells.

Comparative Risk Assessment of Three Native Heliotropium Species in Israel

Pyrrolizidine alkaloids (PAs) are genotoxic carcinogenic phytotoxins mostly prevalent in the Boraginaceae, Asteraceae and Fabaceae families. Heliotropium species (Boraginaceae) are PA-producing weeds, widely distributed in the Mediterranean region, that have been implicated with lethal intoxications in livestock and humans. In Israel, H. europaeum, H. rotundifolium and H. suaveolens are the most prevalent species. The toxicity of PA-producing plants depends on the PA concentration and composition. PAs occur in plants as mixtures of dozens of various PA congeners. Hence, the risk arising from simultaneous exposure to different congeners has to be evaluated. The comparative risk evaluation of the three Heliotropium species was based on recently proposed interim relative potency (iREP) factors, which take into account certain structural features as well as in vitro and in vivo toxicity data obtained for several PAs of different classes. The aim of the present study was to determine the PA profile of the major organ parts of H. europaeum, H. rotundifolium and H. suaveolens in order to assess the plants' relative toxic potential by utilizing the iREP concept. In total, 31 different PAs were found, among which 20 PAs were described for the first time for H. rotundifolium and H. suaveolens. The most prominent PAs were heliotrine-N-oxide, europine-N-oxide and lasiocarpine-N-oxide. Europine-N-oxide displayed significant differences among the three species. The PA levels ranged between 0.5 and 5% of the dry weight. The flowers of the three species were rich in PAs, while the PA content in the root and flowers of H. europaeum was higher than that of the other species. H. europaeum was found to pose a higher risk to mammals than H. rotundifolium, whereas no differences were found between H. europaeum and H. suaveolens as well as H. suaveolens and H. rotundifolium.

Toxic Prediction of Pyrrolizidine Alkaloids and Structure-Dependent Induction of Apoptosis in HepaRG Cells

Pyrrolizidine alkaloids (PAs) are common phytotoxins and could cause liver genotoxicity/carcinogenicity following metabolic activation. However, the toxicity of different structures remains unclear due to the wide variety of PAs. In this study, the absorption, distribution, metabolism, excretion, and toxicity (ADMET) of 40 PAs were analyzed, and their toxicity was predicted by Komputer Assisted Technology (TOPKAT) using Discovery Studio software. The in silico results showed that all PAs except retronecine had good intestinal absorption, and all PAs were predicted to have different toxicity ranges. To verify the predictive results, 4 PAs were selected to investigate cell injury and possible mechanisms of the differentiation in HepaRG cells, including retronecine type of twelve-membered cyclic diester (retrorsine), eleven-membered cyclic diester (monocrotaline), noncyclic diester (retronecine), and platynecine type (platyphylline). After 24 h exposure, retronecine-type PAs exhibited concentration-dependent cytotoxicity. The high-content screening assay showed that cell oxidative stress, mitochondrial damage, endoplasmic reticulum stress, and the concentration of calcium ions increased, and neutral lipid metabolism was changed notably in HepaRG cells. Induced apoptosis by PAs was indicated by cell cycle arrest in the G2/M phase, disrupting the mitochondrial membrane potential. Overall, our study revealed structure-dependent cytotoxicity and apoptosis after PA exposure, suggesting that the prediction results of in silico have certain reference values for compound toxicity. A 1,2-membered cyclic diester seems to be a more potent apoptosis inducer than other PAs.

Lung injury induced by pyrrolizidine alkaloids depends on metabolism by hepatic cytochrome P450s and blood transport of reactive metabolites

Pyrrolizidine alkaloids (PAs) are common phytotoxins with both hepatotoxicity and pneumotoxicity. Hepatic cytochrome P450 enzymes are known to bioactivate PAs into reactive metabolites, which can interact with proteins to form pyrrole-protein adducts and cause intrahepatic cytotoxicity. However, the metabolic and initiation biochemical mechanisms underlying PA-induced pneumotoxicity remain unclear. To investigate the in vivo metabolism basis for PA-induced lung injury, this study used mice with conditional deletion of the cytochrome P450 reductase (Cpr) gene and resultant tissue-selective ablation of microsomal P450 enzyme activities. After oral exposure to monocrotaline (MCT), a pneumotoxic PA widely used to establish animal lung injury models, liver-specific Cpr-null (LCN) mice, but not extrahepatic Cpr-low (xh-CL) mice, had significantly lower level of pyrrole-protein adducts in the serum, liver and lungs compared with wild-type (WT) mice. While MCT-exposed LCN mice had significantly higher blood concentration of intact MCT, compared to MCT-exposed WT or xh-CL mice. Consistent with the MCT in vivo bioactivation data, MCT-induced lung injury, represented by vasculature damage, in WT and xh-CL mice but not LCN mice. Furthermore, reactive metabolites of MCT were confirmed to exist in the blood efflux from the hepatic veins of MCT-exposed rats. Our results provide the first mode-of-action evidence that hepatic P450s are essential for the bioactivation of MCT, and blood circulating reactive metabolites of MCT to the lung causes pneumotoxicity. Collectively, this study presents the scientific basis for the application of MCT in animal lung injury models, and more importantly, warrants public awareness and further investigations of lung diseases associated with exposure to not only MCT but also different PAs.

Comprehensive investigation and risk study on pyrrolizidine alkaloid contamination in Chinese retail honey

Pyrrolizidine alkaloids (PAs) are common phytotoxins. We performed the first comprehensive investigation on PA contamination in Chinese honeys. LC-MS analysis revealed that 58% of 255 honey samples purchased from 17 regions across Mainland China and Taiwan contained PAs with total content ranging over 0.2-281.1 μg/kg. Monocrotaline (from Crotalaria spp), a PA never found in honey in other regions, together with echimidine (Echium plantagineum) and lycopsamine (from Senecio spp.), were three predominant PAs in PA-contaminated Chinese honeys. Further, PAs present in honeys were found to have geographically distinct pattern, indicating possible control of such contamination in future honey production. Moreover, we proposed a new risk estimation approach, which considered both content and toxic potency of individual PAs in honeys, and found that 12% of the PA-contaminated Chinese honeys tested might pose potential health risk. This study revealed a high prevalence and potential health risk of PA contamination in Chinese honeys.

Clinical application of pyrrole-hemoglobin adducts as a biomarker of pyrrolizidine alkaloid exposure in humans

Pyrrolizidine alkaloids (PAs) are naturally occurring hepatotoxins widely present in hundreds of plant species and also known to contaminate many foodstuffs, such as grain, honey, and tea. The formation of pyrrole-protein adducts via metabolic activation of PAs has been suggested as a primary trigger initiating hepatotoxicity. The present study for the first time tested the suitability of pyrrole-hemoglobin adducts as a novel and specific biomarker of PA exposure in humans. The level and elimination kinetics of pyrrole-hemoglobin adducts were systematically investigated in the blood samples of 43 PA-induced liver injury (PA-ILI) patients. The results revealed significantly higher concentrations (84.50 ± 78.38 nM) and longer persistence (~ 4 months) of pyrrole-hemoglobin adducts than that (concentration: 9.53 ± 10.72 nM; persistence: ~ 2 months) of pyrrole-plasma protein adducts, our previously developed PA exposure biomarker. Our findings confirmed that pyrrole-hemoglobin adducts with higher level and longer persistence should serve as a more applicable PA exposure biomarker for future clinical diagnosis of PA-ILI in drug/herb-induced liver injury patients.

Screening of food supplements for toxic pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PA) and PA-N-oxides (PANO) are a group of more than 660 secondary plant metabolites with hepatotoxic, carcinogenic and mutagenic effects in animals and humans. The phytotoxins can enter the food chain by transfer of PA/PANO between plants via the soil, unintended co-harvesting of PA/PANO-producing weeds, as well as by honeybees collecting pollen and nectar of these plants. Thus, bee- and plant-based products, e.g. (herbal) teas, spices and culinary herbs were identified to be a main source of consumers’ exposure to PA/PANO. Consequently, food supplements based on those ingredients may as well be contaminated with PA/PANO, but so far there are only very few studies available on this topic. Therefore, the current study investigated 50 herbal and bee product-based food supplements available in German retail pharmacies, drugstores, and online on the occurrence of 44 PA/PANO. In total, 19 samples contained PA/PANO with sum contents ranging from 0.1 to 105.1 ng/g in solid samples and from 0.03 to 2.20 ng/mL in liquid preparations. Considering the recommended daily consumption, the sum contents were of no or little concern for the health risk of adults, whereas in case of children the contents of singular samples could significantly contribute to the overall PA/PANO exposure.

Structure-Based Reactivity Profiles of Reactive Metabolites with Glutathione

Therapeutic agents can be transformed into reactive metabolites under the action of various metabolic enzymes in vivo and then covalently combine with biological macromolecules (such as protein or DNA), resulting in increasing toxicity. The screening of reactive metabolites in drug discovery and development stages and monitoring of biotransformation in post-market drugs has become an important research field. Generally, reactive metabolites are electrophilic and can be captured by small nucleophiles. Glutathione (GSH) is a small peptide composed of three amino acids (i.e., glutamic acid, cysteine, and glycine). It has a thiol group which can react with electrophilic groups of reactive metabolic intermediates (such as benzoquinone, N-acetyl-p-benzoquinoneimine, and Michael acceptor) to form a stable binding conjugate. This paper aims to provide a review on structure-based reactivity profiles of reactive metabolites with GSH. Furthermore, this review also reveals the relationship between drugs' molecular structures and reactive metabolic toxicity from the perspective of metabolism, giving a reference for drug design and development.

Protein cross-linking in primary cultured mouse hepatocytes by dehydropyrrolizidine alkaloids: Structure-toxicity relationship

Pyrrolizidine alkaloids (PAs) are natural toxins found in about 3%-5% of flowering plants. Dehydropyrrolizidine alkaloids contain a double bond in 1, 2-position of the necine bases, including retronecine type PAs (RET-PAs) and their N-oxides (RET N-oxide-PAs), and otonecine type PAs (OTO-PAs), and are known for their significant hepatotoxicity. Most dehydropyrrolizidine alkaloids are metabolically activated by cytochrome P450 (CYP450) enzymes to generate active pyrroles, which further bind to proteins to form pyrrole-protein adducts (PPAs). Methods for predicting PA-induced liver injury are generally performed on in vitro models with extremely low activities of CYP450 enzymes, which is different from the situation in vivo. In this regard, primary cultured mouse hepatocytes, which showed comparable CYP450 activity with the in vivo models, were applied to illustrate the structure-toxicity relationship of 13 dehydropyrrolizidine alkaloids, namely, eight RET-PAs, three RET N-oxide-PAs, and two OTO-PAs. PA-induced cytotoxicity and PA-generated PPAs were analyzed in primary mouse hepatocytes treated with different PAs. Results showed that PA-induced toxicity was correlated with the amount of PA-generated PPAs. RET-PAs and OTO-PAs were generally more toxic than RET N-oxide-PAs and generated higher amount of PPAs. PPAs were utilized to evaluate the efficiency of metabolic activation and predict the toxic potencies of dehydropyrrolizidine alkaloids. The proposed model could be a new approach for toxicity evaluation and risk control of exposure to PAs.

1-Formyl-7-hydroxy-6,7-dihydro-5H-pyrrolizine (1-CHO-DHP)-Cysteine Conjugates: Metabolic Formation and Binding to Cellular DNA

1-Formyl-7-hydroxy-6,7-dihydro-5H-pyrrolizine (1-CHO-DHP) is a potential proximate carcinogenic metabolite of pyrrolizidine alkaloids. In the present study, we determined that the reaction of 1-CHO-DHP with cysteine generated four identified products. By mass and ¹H NMR spectral analyses, these products are cysteinyl-[2'-S-7]-1-CHO-DHP (P2), cysteinyl-[3'-N-7]-1-CHO-DHP (P3), 7-keto-DHP (P4), and 1-cysteinylimino-DHP (P5). These four compounds were also formed from the incubation of 1-CHO-DHP in HepG2 cells. Compounds P3 and P5 were interconvertible in acetonitrile and water. Incubation of P2 in HepG2 cells generated the four DHP-dG and -dA adducts that we propose to be potential common biomarkers of pyrrolizidine alkaloids exposure and pyrrolizidine alkaloids-induced liver tumor initiation. These four DHP-DNA adducts were also formed from the incubation of a mixture of P3 and P5 in HepG2 cells but not from the incubation with 7-keto-DHP. From the reaction of 1-CHO-DHP with glutathione, only trace amounts of the glutathione-1-CHO-DHP adduct were detected, with the structure unable to be characterized.

Spread of Jacobaea vulgaris and Occurrence of Pyrrolizidine Alkaloids in Regionally Produced Honeys from Northern Germany: Inter- and Intra-Site Variations and Risk Assessment for Special Consumer Groups

Pyrrolizidine alkaloids (PA) and PA N-oxides (PANO) are secondary plant metabolites exhibiting genotoxic and carcinogenic properties. Apart from the roots and leaves, PA/PANO are particularly present in pollen and nectar. Therefore, the spread of Jacobaea vulgaris in certain regions of northern Germany has an impact on the safety of honey produced in that region. In this study, raw honey samples (n = 437) were collected from usually three individual beehives per site (n = 73) in the district of Ostholstein and analyzed for 25 PA/PANO. The results reveal mean levels of 8.4, 1.5, and 72.6 µg/kg and maximum levels of 111, 59.4, and 3313 µg/kg, depending on the season (summer 2015 and spring/summer 2016, respectively). As far as individual data are concerned, sites near areas with J. vulgaris growth did not necessarily result in high PA/PANO values. Furthermore, intra-site investigations revealed remarkable differences in PA/PANO levels of raw honey collected by different bee colonies at the same site. Consumption of these regionally produced honeys entails an increased exposure to PA/PANO, especially in children and high consumers. Margin of exposure values of <10,000 and an exceedance of the health-based guidance value highlight that regionally produced and marketed honey must be considered with care for a proper risk assessment and risk management.

Seneciphylline, a main pyrrolizidine alkaloid in Gynura japonica, induces hepatotoxicity in mice and primary hepatocytes via activating mitochondria-mediated apoptosis

Herbal drug-induced liver injury has been reported worldwide and gained global attention. Thousands of hepatic sinusoidal obstruction syndrome (HSOS) cases have been reported after consumption of herbal medicines and preparations containing pyrrolizidine alkaloids (PAs), which are natural phytotoxins globally distributed. And herbal medicines, such as Gynura japonica, are the current leading cause of PA-induced HSOS. The present study aimed to reveal the mechanism underlying the hepatotoxicity of seneciphylline (Seph), a main PA in G. japonica. Results showed that Seph induced severe liver injury through apoptosis in mice (70 mg/kg Seph, orally) and primary mouse and human hepatocytes (5-50 μM Seph). Further research uncovered that Seph induced apoptosis by disrupting mitochondrial homeostasis, inducing mitochondrial depolarization, mitochondrial membrane potential (MMP) loss, and cytochrome c (Cyt c) release and activating c-Jun N-terminal kinase (JNK). The Seph-induced apoptosis in hepatocytes could be alleviated by Mdivi-1 (50 μM, a dynamin-related protein 1 inhibitor), as well as SP600125 (25 μM, a specific JNK inhibitor) and ZVAD-fmk (50 μM, a general caspase inhibitor). Moreover, the Seph-induced MMP loss in hepatocytes was also rescued by Mdivi-1. In conclusion, Seph induced liver toxicity via activating mitochondrial-mediated apoptosis in mice and primary hepatocytes. Our results provide further information on Seph detoxification and herbal medicines containing Seph such as G. japonica.

Establishment of a novel CYP3A4-transduced human hepatic sinusoidal endothelial cell model and its application in screening hepatotoxicity of pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) are extensively distributed in plants and are known to damage hepatic sinusoidal endothelial cells (HSECs) via metabolic activation mediated by hepatic cytochrome P450 enzymes (CYPs), particularly the CYP3A4 isozyme. Different PAs have distinct toxic potencies and their toxic effects on HSECs are difficult to be determined in cultured cells, because HSECs lack the key CYP3A4 isozyme for metabolic activation. This study aims to establish a novel, convenient and reliable CYP3A4-expressing HSEC model using human HSECs transduced with lentivirus carrying CYP3A4-ires-eGFP, for evaluating the hepatotoxicity of different PAs on their target HSECs. The developed CYP3A4-expressing HSEC (HSEC-CYP3A4) model was verified by the expression of GFP and CYP3A4 and by the ability to metabolize nifedipine, a classic CYP3A4 substrate. Treated with retrorsine, a representative toxic PA, HSEC-CYP3A4 cells showed significantly reduced cell viability, depletion of GSH, and increased formation of pyrrole-protein adducts. Furthermore, this newly developed cell model successfully discriminated the cytotoxic potency of different PAs evidenced by their IC₄₀ values. In conclusion, the established HSEC-CYP3A4 cell model can be used as a rapid screening platform for assessing the relative potencies of individual PAs on their target HSECs and for investigating the mechanisms underlying PA-induced hepatic sinusoidal damage.

Effects of glutathione and cysteine on pyrrolizidine alkaloid-induced hepatotoxicity and DNA adduct formation in rat primary hepatocytes

Pyrrolizidine alkaloids (PAs) are hepatotoxic, genotoxic, and carcinogenic phytochemicals. Upon metabolic activation, PAs produce dehydropyrrolizidine alkaloids (dehydro-PAs) as reactive primary pyrrolic metabolites. Dehydro-PAs are unstable, facilely hydrolyzed to (±)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP). Both dehydro-PAs and DHP are capable of binding to cellular DNA and proteins to form DHP-DNA and DHP-protein adducts leading to tumorigenicity and cytotoxicity. We recently determined that the reaction of dehydro-PAs with glutathione and cysteine generated 7-glutathione-DHP (7-GS-DHP) and 7-cysteine-DHP, respectively which can also bind to DNA to produce DHP-DNA adducts. In this study, we determined the effects of glutathione and cysteine on the induction of hepatocytotoxicity and the formation of DHP-DNA adducts in primary hepatocytes cultured with riddelliine and monocrotaline. We found that both glutathione and cysteine can drastically reduce hepatotoxicity while the levels of DHP-DNA adduct formation are slightly affected.

Absorption difference between hepatotoxic pyrrolizidine alkaloids and their N-oxides - Mechanism and its potential toxic impact

ETHNOPHARMACOLOGICAL RELEVANCE

Pyrrolizidine alkaloids (PAs) are a group of phytotoxins widely present in about 3% of flowering plants. Many PA-containing herbal plants can cause liver injury. Our previous studies demonstrated that PA N-oxides are also hepatotoxic, with toxic potency much lower than the corresponding PAs, due to significant differences in their toxicokinetic fates.

AIM OF STUDY

This study aimed to investigate the oral absorption of PAs and PA N-oxides for better understanding of their significant differences in toxicokinetics and toxic potency.

MATERIALS AND METHODS

The oral absorption of PAs and PA N-oxides in rats and in rat in situ single pass intestine perfusion model was investigated. The intestinal permeability and absorption mechanisms of five pairs of PAs and PA N-oxides were evaluated by using Caco-2 monolayer model.

RESULTS

The plasma concentrations of total PAs and PA N-oxides within 0-60 min were significantly lower in rats orally treated with a PA N-oxide-containing herbal alkaloid extract than with a PA-containing herbal alkaloid extract at the same dose, indicating that the absorption of PA N-oxides was lower than that of PAs. Using the rat in situ single pass intestine perfusion model, less cumulative amounts of retrorsine N-oxide in mesenteric blood were observed compared to that of retrorsine. In Caco-2 monolayer model, all five PAs showed absorption with P_{app AtoB} values [(1.43-16.26) × 10^-6 cm/s] higher than those of corresponding N-oxides with P_{app AtoB} values lower than 1.35 × 10^-6 cm/s. A further mechanistic study demonstrated that except for senecionine N-oxide, retrorsine N-oxide, and lycopsamine N-oxide, all PAs and PA N-oxides investigated were absorbed via passive diffusion. While, for these 3 PA N-oxides, in addition to passive diffusion as their primary transportation, efflux transporter-mediated active transportation was also involved but to a less extent with the efflux ratio of 2.31-3.41. Furthermore, a good correlation between lipophilicity and permeability of retronecine-type PAs and their N-oxides with absorption via passive diffusion was observed, demonstrating that PAs have a better oral absorbability than that of the corresponding PA N-oxides.

CONCLUSION

We discovered that among many contributors, the lower intestinal absorption of PA N-oxides was the initiating contributor that caused differences in toxicokinetics and toxic potency between PAs and PA N-oxides.

A systematic review and quality assessment of case reports of adverse events for borage (Borago officinalis), coltsfoot (Tussilago farfara) and comfrey (Symphytum officinale)

Symphytum officinale (comfrey), Tussilago farfara (coltsfoot) and Borago officinalis (borage) have long histories of therapeutic use, but their safety has been questioned due to the presence of unsaturated pyrrolizidine alkaloids (PAs). The evidence base underlying these concerns relies in part on case reports. This systematic review assesses these case reports for their reliability to inform this debate.

METHOD

Study selection was restricted to case reports describing possible pyrrolizidine alkaloid related harm and ingestion of comfrey, coltsfoot or borage. An extensive search of academic databases was conducted. Papers meeting the criteria were critically appraised.

RESULTS

The search resulted in 11 appropriate case reports, none of which involved borage. Nine reports were assessed for causality and indicated some degree of association between the material ingested and the adverse event. Lack of unequivocal identification of the species ingested compromised attribution and was a significant source of uncertainty. Three levels of identity confusions were found; misidentification or substitution at the level of the whole herb; omission of appropriate botanical identification and attribution of a specific PA to either comfrey or coltsfoot when it is a constituent found in other plants of established toxicity.

CONCLUSION

These cases are an unreliable body of evidence on which to draw conclusions about the safety of the oral consumption of Symphytum officinale and Tussilago farfara. Toxicological studies based on oral ingestion of phytochemically-complex preparations of these herbs may be the most accurate methodology for assessing clinical risk.

Application of the sum parameter method for the determination of pyrrolizidine alkaloids in teas

Tea samples from the Polish market were analysed for the determination of pyrrolizidine alkaloids (PAs). The method applied in the study is based on the GC-MS technique and uses the sum parameter approach for quantification. The analytical protocol was subjected to validation in terms of linearity, repeatability, reproducibility, the limit of quantification and determination, matrix effect, selectivity, and uncertainty, and all evaluated parameters fulfilled the requirements of Commission Decision 2002/657. A total of 78% of the analysed samples contained 1,2-unsaturated PAs, and the concentrations were determined to fall in the range of 2-6498 µg kg^-1. The highly contaminated samples were additionally subjected to LC-MS analysis. Much higher PA concentrations were determined with the sum parameter method, proving its superior quantification capability. The high contamination found prompted an assessment of the dietary exposure from the consumption of infusions prepared from the analysed teas. Two approaches were adopted: one assuming 100% transfer of PAs to the infusions and the second assuming only partial transfer. Regardless of the adopted approach, consumption of several infusions was shown to greatly exceed the maximum acceptable daily intake of PAs.

Clinical characteristics, CT signs, and pathological findings of Pyrrolizidine alkaloids-induced sinusoidal obstructive syndrome: a retrospective study

BACKGROUND

One major etiology of hepatic sinusoidal obstruction syndrome (HSOS) in China is the intake of pyrrolizidine alkaloids (PAs). Since PAs-induced HSOS is a rare disease that has not been clearly characterized until now, the aim of this study was to investigate clinical characteristics, CT features, and pathological findings of PA-induced HSOS.

METHODS

This retrospective cohort study included 116 patients with PAs-induced HSOS and 68 patients with Budd-Chiari syndrome from Jan 2006 to Sep 2016. We collected medical records of the patients, and reviewed image features of CT, and analyzed pathological findings.

RESULTS

Common clinical manifestations of PAs-induced HSOS were abdominal distention (98.26%), ascites (100%), jaundice (52.94%), abdominal pain (36.36%). Abnormal liver function was observed in most of PAs-induced HSOS. On CT scan, common findings included: ascites, hepatomegaly, the thickening of gallbladder wall, pleural effusion, patchy liver enhancement, and heterogeneous hypoattenuation. Most of the patients had a low ascitic total protein (< 25 g/L) and a high SAAG (≥ 11.0 g/L). In acute stage, pathologic features were massive sinusoidal dilatation, sinusoidal congestion, the extravasation of erythrocytes, hepatocellular necrosis, the accumulation of macrophages, the deposition of hemosiderin. In subacute stage, complete loss of pericentral hepatocytes, sinusoidal dilatation, the deposition of pigment granules were observed.

CONCLUSIONS

The PAs-induced HSOS patients displayed distinct clinical characteristics, imaging features, and pathological findings, which provided some evidences for the diagnosis of PAs-induced HSOS.

TRIAL REGISTRATION

ChiCTR-DRD-17010709.

An in vitro study on interaction of anisodine and monocrotaline with organic cation transporters of the SLC22 and SLC47 families

Current study systematically investigated the interaction of two alkaloids, anisodine and monocrotaline, with organic cation transporter OCT1, 2, 3, MATE1 and MATE2-K by using in vitro stably transfected HEK293 cells. Both anisodine and monocrotaline inhibited the OCTs and MATE transporters. The lowest IC₅₀ was 12.9 µmol·L^-1 of anisodine on OCT1 and the highest was 1.8 mmol·L^-1 of monocrotaline on OCT2. Anisodine was a substrate of OCT2 (K_m = 13.3 ± 2.6 µmol·L^-1 and V_max = 286.8 ± 53.6 pmol/mg protein/min). Monocrotaline was determined to be a substrate of both OCT1 (K_m = 109.1 ± 17.8 µmol·L^-1, V_max = 576.5 ± 87.5 pmol/mg protein/min) and OCT2 (K_m = 64.7 ± 14.8 µmol·L^-1, V_max = 180.7 ± 22.0 pmol/mg protein/min), other than OCT3 and MATE transporters. The results indicated that OCT2 may be important for renal elimination of anisodine and OCT1 was responsible for monocrotaline uptake into liver. However neither MATE1 nor MATE2-K could facilitate transcellular transport of anisodine and monocrotaline. Accumulation of these drugs in the organs with high OCT1 expression (liver) and OCT2 expression (kidney) may be expected.

Antibody-based detection of lysine modification of hepatic protein in mice treated with retrorsine

Many pyrrolizidine alkaloids (PAs), an important class of natural products, are hepatotoxic and carcinogenic. Increased attention has been paid to PA poisoning cases worldwide. Generally, most PAs themselves are not toxic. However, reactive intermediates formed from PAs by metabolic oxidation have been linked to toxicity and carcinogenesis. PAs themselves are generally not toxic, and their reactive metabolites resulting from metabolic oxidation are considered to be an essential responsible for PA toxicities. Protein modification by the electrophilic metabolites is proposed to play a key role in PA-induced cytotoxicity. The present study investigated the interaction of lysine residues of proteins with reactive metabolites of toxic PAs. Antibodies selectively recognizing lysine-based protein adduction were prepared and characterized. ELISA and immunoblot methods, in the presence and absence of synthetic model PA adducts, were used to test specific binding of the antibodies to modified lysine residues of BSA and to hepatic proteins extracted from mice treated with retrorsine. The lysine residue adduction was also detected in the tissues of retrorsine-treated mice by use of an immunohistochemical approach. In conclusion, the prepared antibodies selectively recognized the lysine adducts and may be used for the investigation of mechanisms of toxic action of PAs.

Divergent synthesis of N-heterocyclic 1,6-enynes through a zinc-catalyzed decarboxylative A3 reaction

A zinc-catalyzed decarboxylative A³ reaction of cyclic amino acids, α,β-unsaturated aldehydes and terminal alkynes has been developed. A series of functionalized N-heterocyclic 1,6-enynes have been successfully obtained with excellent regioselectivities through this novel approach. In addition, the utility of this straightforward process is demonstrated by the preparation of a polycyclic nitrogen-containing heterocyclic compound.

Preincubation format for a sensitive immunochromatographic assay for monocrotaline, a toxic pyrrolizidine alkaloid

INTRODUCTION

Monocrotaline (MCT), which is classified as a 1,2-dehydropyrrolizidine alkaloid (DHPA), is a toxic compound that is mainly produced by Crotalaria spp. MCT contamination in cereals and herbs leads to hepatitis, gastroenteritis, pulmonary vasculitis and hypertension, and different types of cancer. The current analytical methods for MCT are complicated and expensive using liquid chromatography equipped with mass spectrometry detection.

OBJECTIVE

The aim of this study was to develop a simple and sensitive preincubation format for an immunochromatographic assay (PI-ICA) for MCT detection.

METHODOLOGY

We conducted the PI-ICA via incubation of an MCT-containing sample with an anti-MCT monoclonal antibody conjugated with colloidal gold before strip dipping. We compared the PI-ICA detection sensitivity with that of the conventional ICA (Conv-ICA) format.

RESULTS

The PI-ICA was sensitive with a limit of detection (LOD) of 0.61 ng/mL, which is a 16-fold improvement over the Conv-ICA format. These results indicated that the PI-ICA method exhibits high binding specificity for MCT and low cross-reactivity towards retronecine, retrorsine, senecionine and heliotrine. Sample solutions from plants containing MCT and related DHPAs produced positive results via PI-ICA analysis.

CONCLUSIONS

The proposed PI-ICA system provides a highly sensitive method compared to Conv-ICA. In addition, the developed PI-ICA method is simple and highly effective for detecting MCT contamination.

In vitro metabolism of pyrrolizidine alkaloids - Metabolic degradation and GSH conjugate formation of different structure types

Pyrrolizidine alkaloid (PA) forming plants are found worldwide and may contaminate food products at levels being of concern for human health. Due to the high biodiversity of PA producing plants many different types of PA structures are formed. PAs themselves are not toxic but require metabolic activation to exert toxicity. To investigate if the structure of the PAs affects their in vitro metabolism, we incubated a set of 22 PAs and compared the degradation rates and the amount of formed glutathione (GSH) conjugates. With human liver microsomes, no metabolic degradation of monoesters was found. Degradation rates of diester PAs tended to correlate with their hydrophilicity, whereby the more polar and branched-chained PAs exhibited lower degradation. There was a trend towards higher degradation rates in the presence of rat liver microsomes, but the GSH conjugate levels were similar. Although an effective degradation seems to be related with high GSH conjugate levels, no clear correlation between both parameters could be deduced. For both species no GSH conjugates, or only trace amounts, were formed from monoesters. However, for both open-chained as well as cyclic diesters GSH conjugates were detected and determined levels were comparable for both ester types without major structure-dependent differences.

Prediction of in vivo genotoxicity of lasiocarpine and riddelliine in rat liver using a combined in vitro-physiologically based kinetic modelling-facilitated reverse dosimetry approach

Pyrrolizidine alkaloids (PAs) are naturally occurring genotoxic compounds, and PA-containing plants can pose a risk to humans through contaminated food sources and herbal products. Upon metabolic activation, PAs can form DNA adducts, DNA and protein cross links, chromosomal aberrations, micronuclei, and DNA double-strand breaks. These genotoxic effects may induce gene mutations and play a role in the carcinogenesis of PAs. This study aims to predict in vivo genotoxicity for two well-studied PAs, lasiocarpine and riddelliine, in rat using in vitro genotoxicity data and physiologically based kinetic (PBK) modelling-based reverse dosimetry. The phosphorylation of histone protein H2AX was used as a quantitative surrogate endpoint for in vitro genotoxicity of lasiocarpine and riddelliine in primary rat hepatocytes and human HepaRG cells. The in vitro concentration-response curves obtained from primary rat hepatocytes were subsequently converted to in vivo dose-response curves from which points of departure (PoDs) were derived that were compared to available in vivo genotoxicity data. The results showed that the predicted PoDs for lasiocarpine and riddelliine were comparable to in vivo genotoxicity data. It is concluded that this quantitative in vitro-in silico approach provides a method to predict in vivo genotoxicity for the large number of PAs for which in vivo genotoxicity data are lacking by integrating in vitro genotoxicity assays with PBK modelling-facilitated reverse dosimetry.

Intestinal and hepatic biotransformation of pyrrolizidine alkaloid N-oxides to toxic pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) are among the most significant groups of phytotoxins present in more than 6000 plants in the world. Hepatotoxic retronecine-type PAs and their corresponding N-oxides usually co-exist in plants. Although PA-induced hepatotoxicity is known for a long time and has been extensively studied, the toxicity of PA N-oxide is rarely investigated. Recently, we reported PA N-oxide-induced hepatotoxicity in humans and rodents and also suggested the association of such toxicity with metabolic conversion of PA N-oxides to the corresponding toxic PAs. However, the detailed biochemical mechanism of PA N-oxide-induced hepatotoxicity is largely unknown. The present study investigated biotransformation of four representative cyclic retronecine-type PA N-oxides to their corresponding PAs in both gastrointestinal tract and liver. The results demonstrated that biotransformation of PA N-oxides to PAs was mediated by both intestinal microbiota and hepatic cytochrome P450 monooxygenases (CYPs), in particular CYP1A2 and CYP2D6. Subsequently, the formed PAs were metabolically activated predominantly by hepatic CYPs to form reactive metabolites exerting hepatotoxicity. Our findings delineated, for the first time, that the metabolism-mediated mechanism of PA N-oxide intoxication involved metabolic reduction of PA N-oxides to their corresponding PAs in both intestine and liver followed by oxidative bioactivation of the resultant PAs in the liver to generate reactive metabolites which interact with cellular proteins leading to hepatotoxicity. In addition, our results raised a public concern and also encouraged further investigations on potentially remarkable variations in PA N-oxide-induced hepatotoxicity caused by significantly altered intestinal microbiota due to individual differences in diets, life styles, and medications.

1-Formyl-7-hydroxy-6,7-dihydro-5 H-pyrrolizine (1-CHO-DHP): A Potential Proximate Carcinogenic Metabolite of Pyrrolizidine Alkaloids

Pyrrolizidine alkaloids (PAs) are phytochemicals present in more than 6000 plant species worldwide; about half of the PAs are hepatotoxic, genotoxic, and carcinogenic. Because of their wide exposure and carcinogenicity, the International Programme on Chemical Safety (IPCS) concluded that PAs are a threat to human health and safety. We recently determined that PA-induced liver tumor initiation is mediated by a set of four (±)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5 H-pyrrolizine (DHP)-DNA adducts and proposed that these DHP-DNA adducts are biomarkers of PA exposure and liver tumor initiation. To validate the generality of this metabolic activation pathway and DHP-DNA adducts as biomarkers, it is significant to identify reactive metabolites associated with this metabolic activation pathway. Segall et al. ( Segall et al. ( 1984 ) Drug Metab. Dispos. 12 , 68 - 71 ) previously reported that 1-formyl-7-hydroxy-6,7-dihydro-5 H-pyrrolizine (1-CHO-DHP) is generated from the metabolism of senecionine by mouse liver microsomes. In the present study, we examined the metabolism of seven hepatocarcinogenic PAs (senecionine, intermedine, retrorsine, riddelliine, DHR, heliotrine, and senkirkine) and one noncarcinogenic PA (platyphylline) by human, rat, and mouse liver microsomes. 1-CHO-DHP was identified as a common metabolite from the metabolism of these hepatotoxic PAs, but not from platyphylline. Incubation of 1-CHO-DHP with HepG2 and A549 cells produced the same set of DHP-DNA adducts, which were identified by both LC/MS MRM mode and selected ion monitoring analyses through comparison to synthetic standards. In the incubation medium of 1-CHO-DHP treated HepG2 cells, both DHP and 7-cysteine-DHP were formed, which were capable of binding to cellular DNA to produce DHP-DNA adducts. These results suggest that 1-CHO-DHP is a proximate DNA metabolite of genotoxic and carcinogenic PAs.

Pyrrole-Hemoglobin Adducts, a More Feasible Potential Biomarker of Pyrrolizidine Alkaloid Exposure

Pyrrolizidine alkaloids (PAs) are naturally occurring phytotoxins widely distributed in about 3% of flowering plants. The formation of PA-derived pyrrole-protein adducts is considered as a primary trigger initiating PA-induced hepatotoxicity. The present study aims to (i) further validate our previous established derivatization method using acidified ethanolic AgNO₃ for the analysis of pyrrole-protein adducts and (ii) apply this method to characterize the binding tendency, dose-response, and elimination kinetics of pyrrole-protein adducts in blood samples. Two pyrrole-amino acid conjugates, (±)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5 H-pyrrolizine (DHP)-cysteine (7-cysteine-DHP) and 9-histidine-DHP, were synthesized and used to demonstrate that acidified ethanolic AgNO₃ derivatization can cleave both S-linkage and N-linkage of pyrrole-protein adducts. Subsequently, using precolumn AgNO₃ derivatization followed by ultra-high-pressure liquid chromatography/mass spectrometry analysis, we quantified pyrrole-protein adducts in monocrotaline-treated rat blood protein fractions, including hemoglobin (Hb), plasma, albumin, and plasma residual protein fractions, and found that the amount of pyrrole-Hb adducts was significantly higher than that in all plasma fractions. Moreover, elimination half-life of pyrrole-Hb adducts was also significantly longer than pyrrole-protein adducts in plasma fractions (12.08 vs 2.54-2.93 days). In addition, we also tested blood samples obtained from five PA-induced liver injury patients and found that the amount of pyrrole-protein adducts in blood cells was also remarkably higher than that in plasma. In conclusion, our findings for the first time confirmed that the AgNO₃ derivatization method could be used to measure both S- and N-linked pyrrole-protein adducts and also suggested that pyrrole-Hb adducts with remarkably higher level and longer life span could be a better biomarker of PA exposure.

Development of a two-layer transwell co-culture model for the in vitro investigation of pyrrolizidine alkaloid-induced hepatic sinusoidal damage

Pyrrolizidine alkaloids (PAs) are hepatotoxic and specifically damage hepatic sinusoidal endothelial cells (HSECs) via cytochrome P450 enzymes (CYPs)-mediated metabolic activation. Due to the lack of CYPs in HSECs, currently there is no suitable cell model for investigating PA-induced HSEC injury. This study aimed to establish a two-layer transwell co-culture model that mimics hepatic environment by including HepaRG hepatocytes and HSECs to evaluate cytotoxicity of PAs on their major target HSECs. In this model, PAs were metabolically activated by CYPs in HepaRG hepatocytes to generate reactive pyrrolic metabolites, which react with co-cultured HSECs leading to HSEC damage. Three representative PAs, namely retrorsine, monocrotaline, and clivorine, induced significant concentration-dependent cytotoxicity in HSECs in the co-culture model, but did no cause obvious cytotoxicity directly in HSECs. Using the developed co-cultured model, further mechanism studies of retrorsine-induced HSEC damage demonstrated that the reactive pyrrolic metabolite generated by CYP-mediated bioactivation in HepaRG hepatocytes caused formation of pyrrole-protein adducts, reduction of GSH content, and generation of reactive oxygen species in HSECs, leading to cell apoptosis. The established co-culture model is reliable and applicable for cytotoxic assessment of PA-induced HSEC damage and offers a novel platform for screening toxicity of different PAs on their target cells.

Evidence of Amniotic Epithelial Cell Differentiation toward Hepatic Sinusoidal Endothelial Cells

Amniotic epithelial cells (AECs) represent a useful and noncontroversial source for liver-based regenerative medicine, as they can differentiate into hepatocytes upon transplantation into the liver. However, the possibility that AECs can differentiate into other liver cell types, such as hepatic sinusoidal endothelial cells (HSECs), has never been assessed. In order to test this hypothesis, rat- and human-derived AECs (rAECs and hAECs, respectively) were subjected to endothelial cell tube formation assay in vitro. Moreover, to evaluate differentiation in vivo, the retrorsine (RS) model of liver repopulation was used. Pyrrolizidine alkaloids (including RS) are known to target both hepatocytes and endothelial cells, inducing cell enlargement and inhibition of cell cycle progression. rAECs and hAECs were able to form capillary-like structures when cultured under proangiogenic conditions. For in vivo experiments, rAECs were obtained from dipeptidyl peptidase type IV (DPP-IV, CD26) donors and were transplanted into the liver of recipient CD26 negative animals pretreated with RS. rAEC-derived cells were engrafted in between hepatocytes and resembled HSECs as assessed by morphological analysis and the pattern of expression of CD26. Donor-derived CD26⁺ cells coexpressed HSEC markers RECA-1 and SE-1, while they lacked expression of typical hepatocyte markers (i.e., cytochrome P450, hepatocyte nuclear factor 4α). As such, these results provide the first evidence that AECs can respond to proangiogenic signals in vitro and differentiate into HSECs in vivo. Furthermore, they support the conclusion that AECs possesses great plasticity and represents a promising tool in the field of regenerative medicine both in the liver and in other organs.

Pyrrolizidine Alkaloids: Biosynthesis, Biological Activities and Occurrence in Crop Plants

Pyrrolizidine alkaloids (PAs) are heterocyclic secondary metabolites with a typical pyrrolizidine motif predominantly produced by plants as defense chemicals against herbivores. They display a wide structural diversity and occur in a vast number of species with novel structures and occurrences continuously being discovered. These alkaloids exhibit strong hepatotoxic, genotoxic, cytotoxic, tumorigenic, and neurotoxic activities, and thereby pose a serious threat to the health of humans since they are known contaminants of foods including grain, milk, honey, and eggs, as well as plant derived pharmaceuticals and food supplements. Livestock and fodder can be affected due to PA-containing plants on pastures and fields. Despite their importance as toxic contaminants of agricultural products, there is limited knowledge about their biosynthesis. While the intermediates were well defined by feeding experiments, only one enzyme involved in PA biosynthesis has been characterized so far, the homospermidine synthase catalyzing the first committed step in PA biosynthesis. This review gives an overview about structural diversity of PAs, biosynthetic pathways of necine base, and necic acid formation and how PA accumulation is regulated. Furthermore, we discuss their role in plant ecology and their modes of toxicity towards humans and animals. Finally, several examples of PA-producing crop plants are discussed.

Potentially toxic pyrrolizidine alkaloids in Eupatorium perfoliatum and three related species. Implications for herbal use as boneset

INTRODUCTION

Pro-toxic dehydropyrrolizidine alkaloids are associated with liver disease in humans. The potential for long-term, low-level or intermittent exposures to cause or contribute to chronically-developing diseases is of international concern. Eupatorium perfoliatum is a medicinal herb referred to as boneset. While the presence of dehydropyrrolizidine alkaloids in some Eupatorium species is well-established, reports on Eupatorium perfoliatum are scant and contradictory.

OBJECTIVE

To investigate the presence of dehydropyrrolizidine alkaloids in a survey of boneset samples and related alcoholic tinctures, and hot water infusions and decoctions.

METHODS

Methanol, hot water or aqueous ethanol extracts of Eupatorium perfoliatum and three closely-related species were subjected to HPLC-ESI(+)MS and MS/MS analysis using three complementary column methods. Dehydropyrrolizidine alkaloids were identified from their MS data and comparison with standards.

RESULTS

Forty-nine samples of Eupatorium perfoliatum were shown to contain dehydropyrrolizidine alkaloids (0.0002-0.07% w/w), the majority dominated by lycopsamine and intermedine, their N-oxides and acetylated derivatives. Alcoholic tinctures and hot water infusions and decoctions had high concentrations of the alkaloids. Different chemotypes, hybridisation or contamination of some Eupatorium perfoliatum samples with related species were suggested by the co-presence of retronecine- and heliotridine-based alkaloids.

CONCLUSIONS

Sampling issues, low and high alkaloid chemotypes of Eupatorium perfoliatum or interspecies hybridization could cause the wide variation in dehydropyrrolizidine alkaloid concentrations or the different profiles observed. Concerns associated with dehydropyrrolizidine alkaloids provide a compelling reason for preclusive caution until further research can better define the toxicity and carcinogenicity of the dehydropyrrolizidine alkaloid content of Eupatorium perfoliatum. [Correction added on 12 July 2018, after first online publication: The 'Conclusions' section in the abstract has been added.].

Primary and secondary pyrrolic metabolites of pyrrolizidine alkaloids form DNA adducts in human A549 cells

Humans and animals can be exposed to carcinogenic pyrrolizidine alkaloids (PAs) through consumption of plants commonly found in many parts of the world. Although the liver is the primary target organ for carcinogenic PAs, they have also induced lung tumors in rodents. Hepatic cytochrome P450 activity converts PAs into dehydro-PAs that can be hydrolyzed to dehydropyrrolizidine (DHP); these reactive pyrrolic metabolites can produce four characteristic DNA adducts associated with PA-induced liver tumor initiation in laboratory animals. We reported recently that these four DNA adducts are also formed when 7-glutathione-DHP (7-GS-DHP) or 7-cysteine-DHP is incubated with calf thymus DNA. Here we showed that the four characteristic DNA adducts were formed when human A549 brochoalveolar carcinoma cells were treated with three dehydro-PAs (dehydroriddelliine, dehydromonocrotaline, or dehydroretronecine) or with 7-GS-DHP or 7-cysteine-DHP. For comparison, two parent PAs (riddelliine and monocrotaline) and 7,9-di-glutathionine-DHP were studied. No DHP-DNA adducts were detected with these incubations, confirming that A549 lung carcinoma cells do not express cytochrome P450 enzymes required for metabolic activation of PAs. Our results show that primary and secondary pyrrolic metabolites of carcinogenic PAs produce characteristic DHP-containing DNA adducts in A549 lung cancer cells, suggesting that they are DNA reactive metabolites.

Clinical characteristics and treatment of pyrrolizidine alkaloid-related hepatic vein occlusive disease

BACKGROUND AND AIMS

Hematopoietic stem cell transplantation related hepatic vein occlusive disease (HSCT-HVOD) has been researched extensively; however, little is known about the clinical features and treatment of pyrrolizidine alkaloid-induced HVOD (PA-HVOD). This retrospective single centre study examined the clinical and laboratory characteristics of 108 patients with acute PA-HVOD and explored the efficacy of anticoagulation and TIPS therapy.

METHODS

The study included 108 consecutive patients with PA-HVOD between July 2008 and June 2016. The clinical manifestations and the results of laboratory and imaging tests were evaluated. The survival rates of patients treated with different approaches were recorded.

RESULTS

Serum total bilirubin was <34.2 μmol/L (2 mg/dL) in approximately 40% of patients. More than 90% of patients were presented with hepatomegaly, uneven liver perfusion in the balance phase, compressive stenosis of the hepatic segmental inferior vena cava and decreased peak velocity of portal vein blood flow. Severe portal hypertension was observed in all patients undergoing HVPG examination or TIPS operation. Anticoagulation therapy with low molecular weight heparin combined with warfarin was significantly more effective than liver protection and supportive therapy, and TIPS further improved the prognosis of patients who did not respond to anticoagulation therapy. The total effective rate of the anticoagulation-TIPS ladder therapeutic strategy was 91%.

CONCLUSIONS

Patients with PA-HVOD had different characteristics than those with HSCT-HVOD. Anticoagulation and TIPS treatment may be effective for patients with PA-HVOD.

The role of formation of pyrrole-ATP synthase subunit beta adduct in pyrrolizidine alkaloid-induced hepatotoxicity

Pyrrolizidine alkaloids (PAs) are one of the most significant groups of hepatotoxic phytotoxins. It is well-studied that metabolic activation of PAs generates reactive pyrrolic metabolites that rapidly bind to cellular proteins to form pyrrole-protein adducts leading to hepatotoxicity. Pyrrole-protein adducts all contain an identical core pyrrole moiety regardless of structures of the different PAs; however, the proteins forming pyrrole-protein adducts are largely unknown. The present study revealed that ATP synthase subunit beta (ATP5B), a critical subunit of mitochondrial ATP synthase, was a protein bound to the reactive pyrrolic metabolites forming pyrrole-ATP5B adduct. Using both anti-ATP5B antibody and our prepared anti-pyrrole-protein antibody, pyrrole-ATP5B adduct was identified in the liver of rats, hepatic sinusoidal endothelial cells, and HepaRG hepatocytes treated with retrorsine, a well-studied representative hepatotoxic PA. HepaRG cells were then used to further explore the consequence of pyrrole-ATP5B adduct formation. After treatment with retrorsine, significant amounts of pyrrole-ATP5B adduct were formed in HepaRG cells, resulting in remarkably reduced ATP synthase activity and intracellular ATP level. Subsequently, mitochondrial membrane potential and respiration were reduced, leading to mitochondria-mediated apoptotic cell death. Moreover, pre-treatment of HepaRG cells with a mitochondrial membrane permeability transition pore inhibitor significantly reduced retrorsine-induced toxicity, further revealing that mitochondrial dysfunction caused by pyrrole-ATP5B adduct formation significantly contributed to PA intoxication. Our findings for the first time identified ATP5B as a protein covalently bound to the reactive pyrrolic metabolites of PAs to form pyrrole-ATP5B adduct, which impairs mitochondrial function and significantly contributes to PA-induced hepatotoxicity.

New glutathione conjugate of pyrrolizidine alkaloids produced by human cytosolic enzyme-dependent reactions in vitro

RATIONALE

The toxic metabolites of pyrrolizidine alkaloids (PAs) are initially formed by cytochrome P450-mediated oxidation reactions and primarily eliminated as glutathione (GSH) conjugates. Although the reaction between the reactive metabolites and GSH can occur spontaneously, the role of the cytosolic enzymes in the process has not been studied.

METHODS

The toxic metabolites of selected PAs (retrorsine, monocrotaline, senecionine, lasiocarpine, heliotrine or senkirkine) were generated by incubating them in 100 mM phosphate buffer (pH 7.4) containing liver microsomes of human, pig, rat or sheep, NADPH and reduced GSH in the absence or presence of human, pig, rat or sheep liver cytosolic fraction. The supernatants were analyzed using liquid chromatography connected to Finnigan LTQ ion-trap, Agilent QTOF or Thermo Scientific Q Exactive Focus quadrupole-orbitrap mass spectrometers.

RESULTS

Retrorsine, senecionine and lasiocarpine yielded three GSH conjugates producing [M - H]^- ions at m/z 439 (7-GSH-DHP (CHO)), m/z 441 (7-GSH-DHP (OH)) and m/z 730 (7,9-diGSH-DHP) in the presence of human liver cytosolic fraction. 7-GSH-DHP (CHO) was a novel metabolite. Monocrotaline, heliotrine and senkirkine did not produce this novel 7-GSH-DHP (CHO) conjugate. 7-GSH-DHP (CHO) disappeared when incubated with hydroxylamine, and a new oxime derivative was formed. This metabolite was formed only by the human liver cytosolic enzymes but not in the presence of rat or sheep liver cytosolic fractions under otherwise identical reaction conditions.

CONCLUSIONS

7-GSH-DHP (CHO) has not been reported before, and thus it was considered as a novel metabolite of PAs. This may clarify the mechanisms involved in PA detoxification and widely observed but less understood species differences in response to PA exposure.

Pyrrole-protein adducts - A biomarker of pyrrolizidine alkaloid-induced hepatotoxicity

Pyrrolizidine alkaloids (PAs) are phytotoxins identified in over 6000 plant species worldwide. Approximately 600 toxic PAs and PA N-oxides have been identified in about 3% flowering plants. PAs can cause toxicities in different organs particularly in the liver. The metabolic activation of PAs is catalyzed by hepatic cytochrome P450 and generates reactive pyrrolic metabolites that bind to cellular proteins to form pyrrole-protein adducts leading to PA-induced hepatotoxicity. The mechanisms that pyrrole-protein adducts induce toxicities have not been fully characterized. Methods for qualitative and quantitative detection of pyrrole-protein adducts have been developed and applied for the clinical diagnosis of PA exposure and PA-induced liver injury. This mini-review addresses the mechanisms of PA-induced hepatotoxicity mediated by pyrrole-protein adducts, the analytical methods for the detection of pyrrole-protein adducts, and the development of pyrrole-protein adducts as the mechanism-based biomarker of PA exposure and PA-induced hepatotoxicity.