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

An efficient enzymatic system for studying structure-carcinogenicity relationships: metabolism of pyrrolizidine alkaloids by human liver microsomes in the presence of calf thymus DNA, resulting in the formation of DNA adducts

Pyrrolizidine alkaloids (PAs) form a family of toxic and carcinogenic phytochemicals found in plants worldwide. The metabolism of toxic PAs, both in vivo and in vitro, generates four (±)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-derived DNA adducts, namely, DHP-dG-3, DHP-dG-4, DHP-dA-3, and DHP-dA-4, as documented in previous research. We have proposed that these DHP-DNA adducts play a pivotal role in the induction of liver tumor by PAs in rats and mice, serving as potential common biological biomarkers for PA exposure and carcinogenesis. In this study, we found that the metabolism of PAs and PA N-oxides by human liver microsomes, in the presence of calf thymus DNA, results in the formation of DNA adducts. This process serves as a convenient and biologically significant platform for investigating the structure-carcinogenicity relationships of PAs.

Formation of DHP-DNA Adducts from Rat Liver Microsomal Metabolism of 1,2-Unsaturated Pyrrolizidine Alkaloid-Containing Plant Extracts and Dietary Supplements

1,2-Unsaturated pyrrolizidine alkaloids (PAs) are carcinogenic phytochemicals. We previously determined that carcinogenic PAs and PA N-oxides commonly form a set of four (±)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-DNA adducts, namely, DHP-dG-3, DHP-dG-4, DHP-dA-3, and DHP-dA-4. This set of DHP-DNA adducts has been implicated as a potential biomarker of PA-induced liver tumor initiation from metabolism of individual carcinogenic PAs. To date, it is not known whether this generality occurs from metabolism of PA-containing plant extracts. In this study, we investigate the rat liver microsomal metabolism of nine PA-containing plant extracts and two PA-containing dietary supplements in the presence of calf thymus DNA. The presence of carcinogenic PAs and PA N-oxides in plant extracts was first confirmed by LC-MS/MS analysis with selected reaction monitoring mode. Upon rat liver microsomal metabolism of these PA-containing plant extracts and dietary supplements, the formation of this set of DHP-DNA adducts was confirmed. Thus, these results indicate that metabolism of PA-containing plant extracts and dietary supplements can generate DHP-dG-3, DHP-dG-4, DHP-dA-3, and DHP-dA-4 adducts, thereby potentially initiating liver tumor formation.

Genotoxicity of pyrrolizidine alkaloids in metabolically inactive human cervical cancer HeLa cells co-cultured with human hepatoma HepG2 cells

Pyrrolizidine alkaloids (PAs) are secondary plant metabolites, which can be found as contaminant in various foods and herbal products. Several PAs can cause hepatotoxicity and liver cancer via damaging hepatic sinusoidal endothelial cells (HSECs) after hepatic metabolization. HSECs themselves do not express the required metabolic enzymes for activation of PAs. Here we applied a co-culture model to mimic the in vivo hepatic environment and to study PA-induced effects on not metabolically active neighbour cells. In this co-culture model, bioactivation of PA was enabled by metabolically capable human hepatoma cells HepG2, which excrete the toxic and mutagenic pyrrole metabolites. The human cervical epithelial HeLa cells tagged with H2B-GFP were utilized as non-metabolically active neighbours because they can be identified easily based on their green fluorescence in the co-culture. The PAs europine, riddelliine and lasiocarpine induced micronuclei in HepG2 cells, and in HeLa H2B-GFP cells co-cultured with HepG2 cells, but not in HeLa H2B-GFP cells cultured alone. Metabolic inhibition of cytochrome P450 enzymes with ketoconazole abrogated micronucleus formation. The efflux transporter inhibitors verapamil and benzbromarone reduced micronucleus formation in the co-culture model. Furthermore, mitotic disturbances as an additional genotoxic mechanism of action were observed in HepG2 cells and in HeLa H2B-GFP cells co-cultured with HepG2 cells, but not in HeLa H2B-GFP cells cultured alone. Overall, we were able to show that PAs were activated by HepG2 cells and the metabolites induced genomic damage in co-cultured HeLa cells.

The invasive butterbur contaminates stream and seepage water in groundwater wells with toxic pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) are persistent mutagenic and carcinogenic compounds produced by many common plant species. Health authorities recommend minimising human exposure via food and medicinal products to ensure consumer health and safety. However, there is little awareness that PAs can contaminate water resources. Therefore, no regulations exist to limit PAs in drinking water. This study measured a PA base concentration of ~ 70 ng/L in stream water adjacent to an invasive PA-producing plant Petasites hybridus (Asteraceae). After intense rain the PA concentration increased tenfold. In addition, PAs measured up to 230 ng/L in seepage water from groundwater wells. The dominant PAs in both water types corresponded to the most abundant PAs in the plants (senkirkine, senecionine, senecionine N-oxide). The study presents the first discovery of persistent plant toxins in well water and their associated risks. In addition, it for the first time reports monocrotaline and monocrotaline N-oxide in Petasites sp.

LC/MS study of the diversity and distribution of pyrrolizidine alkaloids in Crotalaria species growing in Colombia

Hepatotoxic and genotoxic pyrrolizidine alkaloids have been involved in the acute poisoning of animals and humans. Crotalaria (Fabaceae) species contain these alkaloids. In this work, the diversity and distribution of pyrrolizidine alkaloids in roots, leaves, flowers, and seeds of Crotalaria pallida, Crotalaria maypurensis, Crotalaria retusa, Crotalaria spectabilis, Crotalaria incana, and Crotalaria nitens were studied. Matrix solid-phase dispersion and ultra-high-performance liquid chromatography coupled with Orbitrap mass spectrometry were successfully employed in pyrrolizidine alkaloids extraction and analysis, respectively. Forty-five pyrrolizidine alkaloids were detected and their identification was based on the mass spectrometry accurate mass measurement and fragmentation pattern analysis. The cyclic retronecine-type diesters monocrotaline, crotaleschenine, integerrimine, usaramine, and their N-oxides were predominantly present. Five novel alkaloids were identified for the first time in Crotalaria species, namely 14-hydroxymonocrotaline, 12-acetylcrotaleschenine, 12-acetylmonocrotaline, 12-acetylintegerrimine, and dihydrointegerrimine. Due to a lack of commercially available standards, the response factor of monocrotaline was used for quantification of pyrrolizidine alkaloids and their N-oxides. Seeds and flowers possessed higher pyrrolizidine alkaloids amounts than roots and leaves. Due to their 1,2-unsaturated pyrrolizidine alkaloids content, the ingestion of Crotalaria plant seeds or other parts through herbal products, infusions, or natural remedies is a serious health threat to humans and livestock.

Evaluation of pyrrolizidine alkaloid-induced genotoxicity using metabolically competent TK6 cell lines

Pyrrolizidine alkaloid (PA)-containing plants are among the most common poisonous plants affecting humans, livestock, and wildlife worldwide. A large number of PAs are known to induce genetic damage after metabolic activation. In the present study, using a battery of fourteen newly developed TK6 cell lines, each expressing a single human cytochrome P450 (CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C18, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5, and 3A7), we identified specific CYPs responsible for bioactivating three PAs - lasiocarpine, riddelliine, and senkirkine. Among the fourteen cell lines, cells expressing CYP3A4 showed significant increases in PA-induced cytotoxicity, evidenced by decreased ATP production and cell viability, and increased caspase 3/7 activities. LC-MS/MS analysis revealed the formation of 1-hydroxymethyl-7-hydroxy-6,7-dihydropyrrolizine (DHP), the main reactive metabolite of PAs, in CYP3A4-expressing TK6 cells. DHP was also detected in CYP3A5- and 3A7-expressing cells after PA exposure, but to a much lesser extent. Subsequently, using a high-throughput micronucleus assay, we demonstrated that PAs induced concentration-dependent increases in micronuclei and G2/M phase cell cycle arrest in three CYP3A variant-expressing TK6 cell lines. Using Western blotting, we observed that PA-induced apoptosis, cell cycle changes, and DNA damage were primarily mediated by CYP3A4. Benchmark dose (BMD) modeling demonstrated that lasiocarpine, of the three PAs, was the most potent inducer of micronuclei, with a BMD₁₀₀ of 0.036 μM. These results indicate that our TK6 cell system holds promise for genotoxicity screening of compounds requiring metabolic activation, identifying specific CYPs involved in bioactivation, and discriminating the genotoxic compounds that have different chemical structures.

Qualitative and quantitative analysis of pyrrolizidine alkaloids for the entire process quality control from Senecio scandens to Senecio scandens-containing preparations by high performance liquid chromatography-tandem mass spectrometry

Senecio scandens as a commonly used traditional Chinese medicine that is used alone or in combination with other herbs in preparations such as QianBai BiYan tablets has attracted much attention because of its hepatotoxic pyrrolizidine alkaloids. Nowadays, most studies for pyrrolizidine alkaloids are only performed on herbs or a preparation, however, production of preparations is a dynamic process, control of toxic impurities for raw materials, or finished products cannot monitor the production process dynamically. Thus, in this study, qualitative and quantitative analysis of pyrrolizidine alkaloids for the entire process quality control from S. scandens to its preparations was carried out with HPLC-MS/MS for the first time, which was more comprehensive and dynamic than the previous single-layer analysis. First, the species of pyrrolizidine alkaloids in S. scandens were analyzed, and the characteristic fragmentation rules of pyrrolizidine alkaloids containing common parent nucleus were found, which can be used to identify these components rapidly in the future. Then, a quantitative method for S. scandens to QianBai BiYan tablets and other nine S. scandens-containing preparations was established, and after the medication safety speculation, all of them met the relevant safety requirements. After that, in order to ensure the stability and controllable of drug quality, the limit of pyrrolizidine alkaloids in preparations was determined according to the safe dosage that is stipulated to be the same as raw materials. Finally, the factors causing the content change of pyrrolizidine alkaloids in S. scandens from different source were studies, which can provide theoretical basis for selecting suitable raw materials for production.

In vitro biotransformation of pyrrolizidine alkaloids in different species: part II-identification and quantitative assessment of the metabolite profile of six structurally different pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PA) exert their toxic effects only after bioactivation. Although their toxicity has already been studied and metabolic pathways including important metabolites were described, the quantification of the latter revealed a large unknown portion of the metabolized PA. In this study, the qualitative and quantitative metabolite profiles of structurally different PAs in rat and human liver microsomes were investigated. Between five metabolites for europine and up to 48 metabolites for lasiocarpine were detected. Proposals for the chemical structure of each metabolite were derived based on fragmentation patterns using high-resolution mass spectrometry. The metabolite profiles of the diester PAs showed a relatively good agreement between both species. The metabolic reactions were summarized into three groups: dehydrogenation, oxygenation, and shortening of necic acid(s). While dehydrogenation of the necine base is considered as bioactivation, both other routes are considered as detoxification steps. The most abundant changes found for open chained diesters were dealkylations, while the major metabolic pathway for cyclic diesters was oxygenation especially at the nitrogen atom. In addition, all diester PAs formed several dehydrogenation products, via the insertion of a second double bond in the necine base, including the formation of glutathione conjugates. In rat liver microsomes, all investigated PAs formed dehydropyrrolizidine metabolites with the highest amount formed by lasiocarpine, whereas in human liver microsomes, these metabolites could only be detected for diesters. Our findings demonstrate that an extensive analysis of PA metabolism can provide the basis for a better understanding of PA toxicity and support future risk assessment.

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.

Intrinsic relative potency of a series of pyrrolizidine alkaloids characterized by rate and extent of metabolism

1,2-Unsaturated pyrrolizidine alkaloids (PAs) are sometimes present in foods or herbal supplements/medicines as impurities and pose potential concerns for liver genotoxicity/carcinogenicity. PAs display a strong structure toxicity relationship, however, current regulatory approaches to risk assessment take the precautionary approach of assuming all PAs display the same potency as the most toxic congeners lasiocarpine (LAS) and riddelliine (RID). Here we explore the relative potencies of a series of structurally diverse PAs by measuring DNA adduct formation in vitro in a rat sandwich culture hepatocyte (SCH) cell system. The adducts generated are consistent with those identified in vivo as biomarkers of PA exposure and potential liver-tumor formation. DNA reactive PAs require metabolic activation to form intermediates that bind DNA, therefore, adduct formation is a direct reflection of reactive metabolite formation. Since the area under the concentration versus time curve (AUC) for the depletion of parent PA from the extracellular media is a measure of PA exposure, the ratio of adducts/AUC provides a measure of hepatocyte exposure to DNA-binding metabolites corresponding to an intrinsic potency for DNA adduct formation. Intrinsic potencies relative to potencies for LAS compare well with existing relative potency data further affirming that PA toxicity varies considerably with chemical structure.