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

The toxicokinetic and metabolism of structurally diverse pyrrolizidine alkaloids in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Pyrrolizidine alkaloids (PAs) are a group of phytotoxins present in about 3% of flowering plants worldwide. Ingestion of PA-containing herbal products may lead to hepatotoxicity. Notably, the toxicokinetic (TK) behaviors, especially pyrrole-protein adducts (PPAs) having the same structure but generated from metabolic activation of different PAs, significantly affect the toxicity of structurally diverse PAs, therefore studying them in their pure form is preferable to extracts to stratify toxic potency of different PAs co-existing in herbal extracts. However, previous studies mainly focus on the establishment of TK profiles of the intact PAs, revealing less or no kinetic information on the main PA metabolites (PA N-oxides) and PPAs which mediate PA-induced hepatotoxicity. In this study, PPA was measured as the biomarker of PA exposure and PA-induced toxicity.

AIM OF STUDY

This study aims to investigate the TK difference between structurally diverse PAs of retronecine-type PAs: retrorsine (RTS) and monocrotaline (MCT), and otonecine-type PA: clivorine (CLI), and their toxicity-related metabolite PPAs and PA N-oxides, the main metabolite of retronecine-type PAs, for the establishment of a more accurate risk assessment of PAs exposure.

MATERIALS AND METHODS

The TK studies were conducted using rats through intravenous (i.v.) or oral (p.o.) administration of PAs at 20 mg/kg. The main TK parameters of PAs and PA N-oxides were determined from plasma concentration-time profiles, and the kinetic profiles of PPAs were assessed from both plasma and erythrocyte concentration-time profiles.

RESULTS

MCT demonstrated the slowest but the highest extent of absorption among the three PAs, while RTS demonstrated a similar absorption rate with a lower extent than CLI. For elimination, MCT demonstrated a similar elimination rate as RTS but the lowest extent of elimination among the three PAs, and CLI exhibited significantly faster elimination than MCT and RTS. Moreover, the formation of PA N-oxide, which only occurs in retronecine-type PAs, was remarkably less in MCT-treated rats compared to RTS-treated ones. Of note, the retronecine-type RTS and MCT induced more PPAs via p.o. than i.v. administration route, whereas the otonecine-type CLI showed the opposite trend.

CONCLUSION

Dramatic TK differences, including not only PAs but also PA N-oxides and the derived protein adduct PPAs, were found among structurally diverse PAs in rats, laying the basis for varied hepatotoxic potencies induced by different PA-containing herbal products. Notably, our findings for the first time uncovered that oral administration of retronecine-type PAs might cause severer toxicity compared with the intravenous route, which warrants further in-depth exploration.

Clinicopathological characteristics and diagnosis of hepatic sinusoidal obstruction syndrome caused by Tusanqi - Case report and literature review

Tusanqi-induced hepatic sinusoidal obstruction syndrome (HSOS) is caused by exposure to pyrrolizidine alkaloids (PAs) and manifests as abdominal distension, liver pain, ascites, jaundice, and hepatomegaly. Pathologically, hepatic congestion and sinusoidal occlusion are observed in HSOS. We summarized the clinical characteristics of 124 patients with HSOS caused by Tusanqi in China between 1980 and 2019, along with those of 831 patients from seven English case series. The main clinical manifestations of PA-HSOS included abdominal pain, ascites, and jaundice. Common imaging features included characteristic heterogeneous density, slender hepatic veins, and other nonspecific changes. The acute stage is primarily manifested as hepatic sinus congestion and necrosis. Meanwhile, the persistence of hepatic sinus congestion and the onset of perisinusoidal fibrosis were observed during the repair stage. Finally, the persistence of hepatic sinusoidal fibrosis and resultant central hepatic vein occlusion were observed in the chronic stage. The new Nanjing standard for PA-HSOS incorporates the history of PA consumption and imaging features and eliminates weight gain and the serum total bilirubin value. Preliminary clinical validation of the Nanjing standard for PA-HSOS diagnosis revealed a sensitivity and specificity of 95.35 and 100%, respectively.

Targeting erythrocyte-mediated hypoxia to alleviate lung injury induced by pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) are widely distributed natural toxins and have been extensively studied for their hepatotoxicity. However, PA-induced pulmonary toxicity remains less studied regarding the initiating mechanism and treatment approaches. Our previous study demonstrated the formation of pyrrole-hemoglobin adducts after PA exposure in vivo, which is suspected to affect the oxygen-carrying capacity of erythrocytes [red blood cells (RBCs)] consequently. The present study aimed to investigate the effects of PAs on the oxygen-carrying capacity of RBCs and the potential of targeting RBC-mediated hypoxia to alleviate PA-induced lung injury. First, rats were treated with retrorsine (RTS) or monocrotaline (MCT) intravenously at 0.2 mmol/kg. The results of Raman spectrometry analysis on blood samples revealed both RTS and MCT significantly reduced the oxygen-carrying capacity of RBCs. Further, MCT (0.2 mmol/kg) was orally given to the rats with or without pretreatment with two doses of erythropoietin (Epo, 500 IU/kg/dose every other day), an RBC-stimulating agent. Biochemical and histological results showed pretreatment with Epo effectively reduced the cardiopulmonary toxicity induced by MCT. These findings provide the first evidence that adduction on hemoglobin, and the resulting RBC damage and impaired oxygen-carrying capacity, are the major initiating mechanism underlying PA-induced pulmonary arterial hypertension (PAH), while targeting the RBC damage is a potential therapeutic approach for PA-induced lung injury.

Inflammation Intensifies Monocrotaline-Induced Liver Injury

Pyrrolizidine alkaloids (PAs) are the most common toxins of plant origin, and it is evident that PAs pollute soil, water, nearby plants, and derived foods. Cases of human poisoning due to ingestion of PA-contaminated foods have been reported in several countries. Monocrotaline (MCT) is a pyrrolizidine alkaloid from the plants of Crotalaria genus that causes hepatic and cardiopulmonary toxicities, and the exhibition of the toxicities requires the metabolic activation by CYP3A4 to form electrophilic dehydro-monocrotaline (DHM). The present study demonstrated that myeloperoxidase (MPO) also participated in the bioactivation of MCT. N-Chloromonocrotaline was detected in both HClO/MCT incubations and MPO/H2O2/MgCl2/MCT incubations. DHM-derived N-acetylcysteine (NAC) conjugates were detected in the above incubations fortified with NAC. Lipopolysaccharide-induced inflammation in mice resulted in an elevated level of hepatic MPO activity, increased metabolic activation of MCT, and intensified elevation of serum ALT and AST activity induced by MCT. MPO inhibitor 4-aminobenzoic acid hydrazide was found to reverse these alterations. Mpo-KO mice were resistant to the observed potentiating effect of inflammation on MCT-induced liver injury. In conclusion, inflammation intensified MCT-induced liver injury. MPO participated in the observed potentiating effect of inflammation on the hepatotoxicity induced by MCT.

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.

Evaluating and predicting the correlations of hepatic concentration and pyrrole-protein adduction with hepatotoxicity induced by retrorsine based on pharmacokinetic/pharmacodynamic model

Retrosine (RTS) is a pyrrolozidine alkaloid and a known hepatotoxin that widely exist in nature. The mechanisms involved in toxic action of pyrrolizidine alkaloids need further investigation. The objective of the present study was to evaluate the correlation of RTS hepatotoxicity with hepatic RTS concentration and pyrrole-protein adduction. Mice were intragastrically treated with RTS alone or RTS and ketoconazole (KTZ) simultaneously. Sera and liver tissues were collected at various time points after administration, followed by the determination of changes in serum transaminase activity, hepatic RTS concentration and pyrrole-protein adduction. The correlation of RTS hepatotoxicity with hepatic RTS concentration and hepatic pyrrole-protein adduction were examined by use of Sigmoid-Emax PK/PD models. Dose-dependent hepatotoxicity, hepatic RTS concentration and pyrrole-protein adduction were observed in the animals, which could be modulated by co-treatment with KTZ. The fit parameters indicated pyrrole-protein adduction was more closely related with liver injury than hepatic RTS concentration. Similar correlation was observed in mice given low-dose of RTS for 4 consecutive days. RTS hepatotoxicity is correlated with hepatic pyrrole-protein adduction derived from RTS rather than hepatic RTS concentration. The observed protein modification would be a good indicator to predict the hepatoxicity of RTS at low dose.

Integrative Metabolomics and Proteomics Detected Hepatotoxicity in Mice Associated with Alkaloids from Eupatorium fortunei Turcz

The traditional Chinese herbal medicine Eupatorium fortunei Turcz. (E. fortunei) has been widely adopted to treat nausea, diabetes, siriasis, and poor appetite. However, E. fortunei contains multiple pyrrolizidine alkaloids (PAs). This study aimed to investigate the hepatotoxicity of total alkaloids in E. fortunei (EFTAs) and identify the toxic mechanisms of EFTAs on hepatocytes. Liquid chromatography with a tandem mass spectrometry assay with reference standards indicated that EFTAs mainly consisted of eight PAs whose content accounted for 92.38% of EFTAs. EFTAs markedly decreased mouse body and liver weights and increased the contents of AST and ALT. The histopathological assays demonstrated that, after exposition to EFTAs, the structures of hepatocytes were damaged and the fibrosis and apoptosis in hepatocytes were accelerated. Moreover, EFTAs increased the serum level of inflammatory cytokines and aggravated circulating oxidative stress. A combination of hepatic proteomics and metabolomics was used to investigate the toxic mechanisms of EFTAs. The study revealed that EFTAs seriously disrupted glycerophospholipid metabolism by upregulating the contents of lysophosphatidylglycerol acyltransferase 1 and phosphatidylinositol and downregulating the contents of choline/ethanolamine kinase beta, choline-ethanolamine phosphotransferase 1, phospholipase D4, 1-acylglycerophosphocholine, phosphatidylcholine, and dihydroxyacetone phosphate in the liver, resulting in detrimental inflammation, fibrosis, and apoptosis. This study revealed that EFTAs induced severe hepatotoxicity by disrupting glycerophospholipid metabolism.

Electrochemiluminescence sensor for point-of-care detection of pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) and PA N-oxides are hepatotoxic natural products, produced by over 6000 plant species worldwide. However, an unmet need remains for confirmative measurement of PAs in routine clinical tests. Here, we develop a visual, easy-to-use, and economic mesoporous silica-electrochemiluminescence (MPS-ECL) sensor for point-of-care (POC) testing of PAs, utilizing MPS's amplification effect on positive ions. The relationship between PAs' different structures and corresponding Ru(bpy)₃²⁺ ECL activity shows that reaction mechanism, stability of intermediate, molecular geometry and alternative anodic reactivity significantly affect the ECL activity. The ECL intensity varies among different PAs: monocrotaline _˃ senecionine N-oxide _˃ retrorsine _˃ senkirkine. The POC sensors possess excellent linearity (0.9993 > R² > 0.9944), low detection limits (0.02 μM-0.07 μM), and good recoveries (90.12%-105.93%), indicating good accuracy and practicability. The portable and low-cost sensor is user-friendly, which holds promise to be applied to POC testing of PAs in drugs, food products, and clinical samples, which is promising for initial assessments of PA-induced health risk.

Liquorice Extract and 18β-Glycyrrhetinic Acid Protect Against Experimental Pyrrolizidine Alkaloid-Induced Hepatotoxicity in Rats Through Inhibiting Cytochrome P450-Mediated Metabolic Activation

Misuse of pyrrolizidine alkaloid (PA)-containing plants or consumption of PA-contaminated foodstuffs causes numerous poisoning cases in humans yearly, while effective therapeutic strategies are still limited. PA-induced liver injury was initiated by cytochrome P450 (CYP)-mediated metabolic activation and subsequent formation of adducts with cellular proteins. Liquorice, a hepato-protective herbal medicine, is commonly used concurrently with PA-containing herbs in many compound traditional Chinese medicine formulas, and no PA-poisoning cases have been reported with this combination. The present study aimed to investigate hepato-protective effects of liquorice aqueous extract (EX) and 18β-glycyrrhetinic acid (GA, the primary bioactive constituent of liquorice) against PA-induced hepatotoxicity and the underlying mechanism. Histopathological and biochemical analysis demonstrated that both single- and multiple-treatment of EX (500 mg/kg) or GA (50 mg/kg) significantly attenuated liver damage caused by retrorsine (RTS, a representative hepatotoxic PA). The formation of pyrrole-protein adducts was significantly reduced by single- (30.3% reduction in liver; 50.8% reduction in plasma) and multiple- (32.5% reduction in liver; 56.5% reduction in plasma) treatment of GA in rats. Single- and multiple-treatment of EX also decreased the formation of pyrrole-protein adducts, with 30.2 and 31.1% reduction in rat liver and 51.8 and 53.1% reduction in rat plasma, respectively. In addition, in vitro metabolism assay with rat liver microsomes demonstrated that GA reduced the formation of metabolic activation-derived pyrrole-glutathione conjugate in a dose-dependent manner with the estimated IC₅₀ value of 5.07 µM. Further mechanism study showed that GA inhibited activities of CYPs, especially CYP3A1, the major CYP isoform responsible for the metabolic activation of RTS in rats. Enzymatic kinetic study revealed a competitive inhibition of rat CYP3A1 by GA. In conclusion, our findings demonstrated that both EX and GA exhibited significant hepato-protective effects against RTS-induced hepatotoxicity, mainly through the competitive inhibition of CYP-mediated metabolic activation of RTS.

The key role of gut-liver axis in pyrrolizidine alkaloid-induced hepatotoxicity and enterotoxicity

Pyrrolizidine alkaloids (PAs) are the most common phytotoxins with documented human hepatotoxicity. PAs require metabolic activation by cytochromes P450 to generate toxic intermediates which bind to proteins and form protein adducts, thereby causing cytotoxicity. This study investigated the role of the gut-liver axis in PA intoxication and the underlying mechanisms. We exposed mice to retrorsine (RTS), a representative PA, and for the first time found RTS-induced intestinal epithelium damage and disruption to intestinal barrier function. Using mice with tissue-selective ablation of P450 activity, we found that hepatic P450s, but not intestinal P450s, were essential for PA bioactivation. Besides, in RTS-exposed, bile duct-cannulated rats, we found the liver-derived reactive PA metabolites were transported by bile into the intestine to exert enterotoxicity. The impact of gut-derived pathogenic factors in RTS-induced hepatotoxicity was further studied in mice with dextran sulfate sodium (DSS)-induced chronic colitis. DSS treatment increased the hepatic endotoxin level and depleted hepatic reduced glutathione, thereby suppressing the PA detoxification pathway. Compared to RTS-exposed normal mice, the colitic mice displayed more severe RTS-induced hepatic vasculature damage, fibrosis, and steatosis. Overall, our findings provide the first mode-of-action evidence of PA-induced enterotoxicity and highlight the importance of gut barrier function in PA-induced liver injury.

Fasting augments pyrrolizidine alkaloid-induced hepatotoxicity

Pyrrolizidine alkaloids (PAs) are phytotoxins widely present in various natural products and foodstuffs. The present study aims to investigate the effects of fasting on PA-induced hepatotoxicity and the underlying biochemical mechanisms. The results of hepatotoxic study showed that 15-h overnight fasting significantly exacerbated the hepatotoxicity of retrorsine (RTS, a representative toxic PA) in fasted rats compared to fed rats, as indicated by remarkably elevated plasma ALT and bilirubin levels and obvious liver histological changes. Further toxicokinetic studies revealed that fasting significantly enhanced cytochromes P450 enzymes (CYPs)-mediated metabolic activation of RTS leading to increased formation of pyrrole-protein adducts and thus decreased the in vivo exposure and excretion of both parent RTS and its N-oxide metabolite. Metabolic studies demonstrated that fasting induced enzyme activities of CYP1A2, CYP2B6 and CYP2E1 that participated in catalyzing RTS to its reactive pyrrolic metabolites. Moreover, fasting also dramatically decreased hepatic glutathione (GSH) content, which restricted the detoxification of GSH by neutralizing the reactive pyrrolic metabolite of RTS, further contributing to the enhanced hepatotoxicity. The present findings may have an impact on future PA toxicity tests with different dietary styles and/or risk assessment of metabolite-mediated toxins by considering the profound effects of fasting.

Quo vadis blood protein adductomics?

Chemicals are measured regularly in air, food, the environment, and the workplace. Biomonitoring of chemicals in biological fluids is a tool to determine the individual exposure. Blood protein adducts of xenobiotics are a marker of both exposure and the biologically effective dose. Urinary metabolites and blood metabolites are short term exposure markers. Stable hemoglobin adducts are exposure markers of up to 120 days. Blood protein adducts are formed with many xenobiotics at different sites of the blood proteins. Newer methods apply the techniques developed in the field of proteomics. Larger adducted peptides with 20 amino acids are used for quantitation. Unfortunately, at present the methods do not reach the limits of detection obtained with the methods looking at single amino acid adducts or at chemically cleaved adducts. Therefore, to progress in the field new approaches are needed.

Metabolic Activation of the Toxic Natural Products From Herbal and Dietary Supplements Leading to Toxicities

Currently, herbal and dietary supplements have been widely applied to prevent and treat various diseases. However, the potential toxicities and adverse reactions of herbal and dietary supplements have been increasingly reported, and have gradually attracted widespread attention from clinical pharmacists and physicians. Metabolic activation of specific natural products from herbal and dietary supplements is mediated by hepatic cytochrome P450 or intestinal bacteria, and generates chemical reactive/toxic metabolites that bind to cellular reduced glutathione or macromolecules, and form reactive metabolites-glutathione/protein/DNA adducts, and these protein/DNA adducts can result in toxicities. The present review focuses on the relation between metabolic activation and toxicities of natural products, and provides updated, comprehensive and critical comment on the toxic mechanisms of reactive metabolites. The key inductive role of metabolic activation in toxicity is highlighted, and frequently toxic functional groups of toxic natural products were summarized. The biotransformation of drug cytochrome P450 or intestinal bacteria involved in metabolic activation were clarified, the reactive metabolites-protein adducts were selected as biomarkers for predicting toxicity. And finally, further perspectives between metabolic activation and toxicities of natural products from herbal and dietary supplements are discussed, to provide a reference for the reasonable and safe usage of herbal and dietary supplements.

Pyrrolizidine Alkaloid-Induced Hepatotoxicity Associated with the Formation of Reactive Metabolite-Derived Pyrrole-Protein Adducts

Pyrrolizidine alkaloids (PAs) with 1,2-unsaturated necine base are hepatotoxic phytotoxins. Acute PA intoxication is initiated by the formation of adducts between PA-derived reactive pyrrolic metabolites with cellular proteins. The present study aimed to investigate the correlation between the formation of hepatic pyrrole-protein adducts and occurrence of PA-induced liver injury (PA-ILI), and to further explore the use of such adducts for rapidly screening the hepatotoxic potency of natural products which contain PAs. Aqueous extracts of Crotalaria sessiliflora (containing one PA: monocrotaline) and Gynura japonica (containing two PAs: senecionine and seneciphylline) were orally administered to rats at different doses for 24 h to investigate PA-ILI. Serum alanine aminotransferase (ALT) activity, hepatic glutathione (GSH) level, and liver histological changes of the treated rats were evaluated to assess the severity of PA-ILI. The levels of pyrrole-protein adducts formed in the rats' livers were determined by a well-established spectrophotometric method. The biological and histological results showed a dose-dependent hepatotoxicity with significantly different toxic severity among groups of rats treated with herbal extracts containing different PAs. Both serum ALT activity and the amount of hepatic pyrrole-protein adducts increased in a dose-dependent manner. Moreover, the elevation of ALT activity correlated well with the formation of hepatic pyrrole-protein adducts, regardless of the structures of different PAs. The findings revealed that the formation of hepatic pyrrole-protein adducts-which directly correlated with the elevation of serum ALT activity-was a common insult leading to PA-ILI, suggesting a potential for using pyrrole-protein adducts to screen hepatotoxicity and rank PA-containing natural products, which generally contain multiple PAs with different structures.

Metabolic Toxification of 1,2-Unsaturated Pyrrolizidine Alkaloids Causes Human Hepatic Sinusoidal Obstruction Syndrome: The Update

Saturated and unsaturated pyrrolizidine alkaloids (PAs) are present in more than 6000 plant species growing in countries all over the world. They have a typical heterocyclic structure in common, but differ in their potential toxicity, depending on the presence or absence of a double bond between C1 and C2. Fortunately, most plants contain saturated PAs without this double bond and are therefore not toxic for consumption by humans or animals. In a minority of plants, however, PAs with this double bond between C1 and C2 exhibit strong hepatotoxic, genotoxic, cytotoxic, neurotoxic, and tumorigenic potentials. If consumed in error and in large emouns, plants with 1,2-unsaturated PAs induce metabolic breaking-off of the double bonds of the unsaturated PAs, generating PA radicals that may trigger severe liver injury through a process involving microsomal P450 (CYP), with preference of its isoforms CYP 2A6, CYP 3A4, and CYP 3A5. This toxifying CYP-dependent conversion occurs primarily in the endoplasmic reticulum of the hepatocytes equivalent to the microsomal fraction. Toxified PAs injure the protein membranes of hepatocytes, and after passing their plasma membranes, more so the liver sinusoidal endothelial cells (LSECs), leading to life-threatening hepatic sinusoidal obstruction syndrome (HSOS). This injury is easily diagnosed by blood pyrrolizidine protein adducts, which are perfect diagnostic biomarkers, supporting causality evaluation using the updated RUCAM (Roussel Uclaf Causality Assessment Method). HSOS is clinically characterized by weight gain due to fluid accumulation (ascites, pleural effusion, and edema), and may lead to acute liver failure, liver transplantation, or death. In conclusion, plant-derived PAs with a double bond between C1 and C2 are potentially hepatotoxic after metabolic removal of the double bond, and may cause PA-HSOS with a potential lethal outcome, even if PA consumption is stopped.

Developing urinary pyrrole-amino acid adducts as non-invasive biomarkers for identifying pyrrolizidine alkaloids-induced liver injury in human

Pyrrolizidine alkaloids (PAs) have been found in over 6000 plants worldwide and represent the most common hepatotoxic phytotoxins. Currently, a definitive diagnostic method for PA-induced liver injury (PA-ILI) is lacking. In the present study, using a newly developed analytical method, we identified four pyrrole-amino acid adducts (PAAAs), namely pyrrole-7-cysteine, pyrrole-9-cysteine, pyrrole-9-histidine, and pyrrole-7-acetylcysteine, which are generated from reactive pyrrolic metabolites of PAs, in the urine of PA-treated male Sprague Dawley rats and PA-ILI patients. The elimination profiles, abundance, and persistence of PAAAs were systematically investigated first in PA-treated rat models via oral administration of retrorsine at a single dose of 40 mg/kg and multiple doses of 5 mg/kg/day for 14 consecutive days, confirming that these urinary excreted PAAAs were derived specifically from PA exposure. Moreover, we determined that these PAAAs were detected in ~ 82% (129/158) of urine samples collected from ~ 91% (58/64) of PA-ILI patients with pyrrole-7-cysteine and pyrrole-9-histidine detectable in urine samples collected at 3 months or longer times after hospital admission, indicating adequate persistence time for use as a clinical test. As direct evidence of PA exposure, we propose that PAAAs can be used as a biomarker of PA exposure and the measurement of urinary PAAAs could be used as a non-invasive test assisting the definitive diagnosis of PA-ILI in patients.

Characterization of liver injury induced by a pyrrolizidine alkaloid in rats

BACKGROUND

Pyrrolizidine alkaloids (PAs) are common phytotoxins. PA intoxication is reported to cause severe acute liver damage, typically known as hepatic sinusoidal obstruction syndrome (HSOS), but it remains obscure whether the acute liver damage may progress into chronic liver disease characterized by hepatic fibrosis.

PURPOSE

This study aims to characterize the biochemical markers of liver injury and histological features of regressive and progressive liver fibrosis, and to examine changes in hepatic gene expression that may underpin mechanisms of fibrogenesis in rats induced by retrorsine (RTS), a representative toxic PA.

STUDY DESIGN/METHODS

Rats were gavaged with RTS via two dosing regimens, i.e. a single dose of 40 mg/kg (Group 1) and two doses of 40 mg/kg and 20 mg/kg on day 0 and day 7 (Group 2), respectively. Rats receiving one (Group 3) or two (Group 4) doses of vehicle served as negative controls. The animals were followed for up to 16 weeks by serum biochemical analyses and histological examination, and gene expression assays of liver tissues.

RESULTS

Acute liver injury on day 2 manifested as HSOS, characterized by sinusoidal dilation, endothelial cell damage, and elevated serum alanine aminotransferase activity and bilirubin levels. In Group 1, mild liver fibrosis developed at sinusoids and perisinusoidal space surrounding the central veins at week 1 and 2, and thereafter, all liver injury resolved gradually. In Group 2, liver fibrosis progressed within the 16-week observation period. No apparent liver injury was observed in Groups 3 and 4. Compared with negative control groups, RTS induced myofibroblastic activation, TGF-β1 signaling, and changes in expression of matrix metalloproteinase 9 (MMP-9) and tissue inhibitor of metalloproteinase 1 (TIMP-1). These dynamic changes differed in Groups 1 and 2, corresponding with the regression and progression of liver fibrosis, respectively, in these groups.

CONCLUSION

This study has provided in-vivo proof of concept that "one hit" and "two hits" of RTS lead to acute resolving liver injury and chronic progressive liver fibrosis, respectively. These animal models may serve as powerful tools for studying RTS toxicology and related preventive and therapeutic strategies and as positive controls for studying other PA- and non-PA-induced liver injury.

Tu-San-Qi (Gynura japonica): the culprit behind pyrrolizidine alkaloid-induced liver injury in China

Herbs and dietary supplement-induced liver injury (HILI) is the leading cause of drug-induced liver injury in China. Among different hepatotoxic herbs, the pyrrolizidine alkaloid (PA)-producing herb Gynura japonica contributes significantly to HILI by inducing hepatic sinusoidal obstruction syndrome (HSOS), a liver disorder characterized by hepatomegaly, hyperbilirubinemia, and ascites. In China, G. japonica has been used as one of the plant species for Tu-San-Qi and is often misused with non-PA-producing Tu-San-Qi (Sedum aizoon) or even San-Qi (Panax notoginseng) for self-medication. It has been reported that over 50% of HSOS cases are caused by the intake of PA-producing G. japonica. In this review, we provide comprehensive information to distinguish these Tu-San-Qi-related herbal plant species in terms of plant/medicinal part morphologies, medicinal indications, and chemical profiles. Approximately 2156 Tu-San-Qi-associated HSOS cases reported in China from 1980 to 2019 are systematically reviewed in terms of their clinical manifestation, diagnostic workups, therapeutic interventions, and outcomes. In addition, based on the application of our developed mechanism-based biomarker of PA exposure, our clinical findings on the definitive diagnosis of 58 PA-producing Tu-San-Qi-induced HSOS patients are also elaborated. Therefore, this review article provides the first comprehensive report on 2214 PA-producing Tu-San-Qi (G. japonica)-induced HSOS cases in China, and the information presented will improve public awareness of the significant incidence of PA-producing Tu-San-Qi (G. japonica)-induced HSOS and facilitate future prevention and better clinical management of this severe HILI.

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.

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.

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.

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.

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.

Pulmonary toxicity is a common phenomenon of toxic pyrrolizidine alkaloids

The hepatotoxic pyrrolizidine alkaloids (PAs) are metabolically activated in the liver to form reactive dehydro-PAs, which generate pyrrole-protein adducts leading to hepatotoxicity. Monocrotaline, but not other PAs, is also pneumotoxic, supposedly due to the migration of the liver-generated corresponding dehydro-PA into the lung to form pyrrole-protein adducts to induce pneumotoxicity. The present study investigated whether other PAs are also pneumotoxic. Metabolic activation of four representative hepatotoxic PAs, monocrotaline, retrorsine, riddelliine and clivorine, was investigated using rat liver or lung S9 incubation. All PAs produced pyrrole-protein adducts significantly in rat liver S9 but negligible in lung S9 fraction, revealing that liver is the key organ responsible for metabolic activation generating dehydro-PAs. Furthermore, these four PAs and another two PAs present in the alkaloid extract of Gynura segetum, a widely used PA-producing herb responsible for human PA poisonings in China, were orally administered to rats using the same hepatotoxic dose of 0.2 mmol/kg. All six PAs induced pneumotoxicity in rats within 48 h. The results demonstrated that pneumotoxicity could be a common phenomenon of PAs and the liver-derived dehydro-PAs might move to the lung and form pyrrole-protein adducts, leading to pulmonary toxicity.

The long persistence of pyrrolizidine alkaloid-derived pyrrole-protein adducts in vivo: Kinetic study following multiple exposures of a pyrrolizidine alkaloid containing extract of Gynura japonica

Gynura japonica (also named Tusanqi in Chinese) is used as a folk herbal medicine for treating blood stasis or traumatic injury. However, hundreds of hepatic sinusoidal obstruction syndrome (HSOS) cases have been reported after consumption of preparations made from G. japonica because it contains large amounts of hepatotoxic pyrrolizidine alkaloids (PAs). To date, blood pyrrole-protein adducts (PPAs) are suggested as biomarkers for the diagnosis of PA-induced HSOS in clinics. However, the concentration of PPAs in the blood is greatly affected by several factors including the amount of PA exposure, herb intake period, and blood sampling time after the last exposure. In present study, the kinetic characters of PPAs in serum and liver as well as other potential target organs were studied systematically and comprehensively following multiple exposures of PAs in G. japonica extract (GJE). As results, PPAs content reached to a plateau both in serum and liver after the mice were treated with GJE for 2 weeks on daily basis. PPAs cleared significantly slower in liver (T_1/2ke∼184.6 h, ∼7.7 days) than in serum (T_1/2ke∼95.8 h, ∼4.0 days). Although more than 90 % PPAs were removed 2 weeks after the last dosing, PPAs still persisted in the liver until the end of the experiment, i.e. 8 weeks after the last dosing. The results would be of great help for understanding the importance of PPAs for PA-induced toxicity and its detoxification.

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.