Pyrrolizidine alkaloids (PAs) and PA N-oxides are a class of phytochemical carcinogens contained in over 6000 plant species spread around the world. It has been estimated that approximately half of ...the 660 PAs and PA N-oxides that have been characterized are cytotoxic, genotoxic, and tumorigenic. It was recently determined that a genotoxic mechanism of liver tumor initiation mediated by PA-derived DNA adducts is a common metabolic activation pathway of a number of PAs. We proposed this set of PA-derived DNA adducts could be a common biological biomarker of PA exposure and a potential biomarker of PA-induced liver tumor formation. We have also found that several reactive secondary pyrrolic metabolites can dissociate and interconvert to other secondary pyrrolic metabolites, resulting in the formation of the same exogenous DNA adducts. This present perspective reports the current progress on these new findings and proposes future research needed for obtaining a greater understanding of the role of this activation pathway and validating the use of this set of PA-derived DNA adducts as a biological biomarker of PA-induced liver tumor initiation.
Nanotechnology is a rapidly developing field in the 21st century, and the commercial use of nanomaterials for novel applications is increasing exponentially. To date, the scientific basis for the ...cytotoxicity and genotoxicity of most manufactured nanomaterials are not understood. The mechanisms underlying the toxicity of nanomaterials have recently been studied intensively. An important mechanism of nanotoxicity is the generation of reactive oxygen species (ROS). Overproduction of ROS can induce oxidative stress, resulting in cells failing to maintain normal physiological redox-regulated functions. This in turn leads to DNA damage, unregulated cell signaling, change in cell motility, cytotoxicity, apoptosis, and cancer initiation. There are critical determinants that can affect the generation of ROS. These critical determinants, discussed briefly here, include: size, shape, particle surface, surface positive charges, surface-containing groups, particle dissolution, metal ion release from nanometals and nanometal oxides, UV light activation, aggregation, mode of interaction with cells, inflammation, and pH of the medium.
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
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-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
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-oxide is rarely investigated. Recently, we reported PA
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-oxide-induced hepatotoxicity in humans and rodents and also suggested the association of such toxicity with metabolic conversion of PA
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-oxides to the corresponding toxic PAs. However, the detailed biochemical mechanism of PA
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-oxide-induced hepatotoxicity is largely unknown. The present study investigated biotransformation of four representative cyclic retronecine-type PA
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-oxides to their corresponding PAs in both gastrointestinal tract and liver. The results demonstrated that biotransformation of PA
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-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
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-oxide intoxication involved metabolic reduction of PA
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-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
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-oxide-induced hepatotoxicity caused by significantly altered intestinal microbiota due to individual differences in diets, life styles, and medications.
Background and Aims
Mutational signature analyses are an effective tool in identifying cancer etiology. Humans are frequently exposed to pyrrolizidine alkaloids (PAs), the most common carcinogenic ...phytotoxins widely distributed in herbal remedies and foods. However, due to the lack of human epidemiological data, PAs are classified as group II hepatocarcinogens by the World Health Organization. This study identified a PA mutational signature as the biomarker to investigate the association of PA exposure with human liver cancer.
Approach and Results
Pyrrole‐protein adducts (PPAs), the PA exposure biomarker, were measured and found in 32% of surgically resected specimens from 34 patients with liver cancer in Hong Kong. Next, we delineated the mode of mutagenic and tumorigenic actions of retrorsine, a representative PA, in mice and human hepatocytes (HepaRG). Retrorsine induced DNA adduction, DNA damage, and activation of tumorigenic hepatic progenitor cells, which initiated hepatocarcinogenesis. PA mutational signature, as the unique molecular fingerprint of PA‐induced mutation, was derived from exome mutations in retrorsine‐exposed mice and HepaRG cells. Notably, PA mutational signature was validated in genomes of patients with PPA‐positive liver cancer but not patients with PPA‐negative liver cancer, confirming the specificity of this biomarker in revealing PA‐associated liver cancers. Furthermore, we examined the established PA mutational signature in 1,513 liver cancer genomes and found that PA‐associated liver cancers were potentially prevalent in Asia (Mainland China 48%, Hong Kong 44%, Japan 22%, South Korea 6%, Southeast Asia 25%) but minor in Western countries (North America 3% and Europe 5%).
Conclusions
This study provides a clinical indication of PA‐associated liver cancer. We discovered an unexpectedly extensive implication of PA exposure in patients with liver cancer, laying the scientific basis for precautionary approaches and prevention of PA‐associated human liver cancers.
In the recent decades, the use of herbal products has been rapidly growing in the Western countries. While their use in many cases causes adverse effects, to date, safety issues of herbal products ...have not been adequately addressed. It is rarely determined whether the non-purported bioactive constituents in the herbs and the metabolites of the bioactive components can lead to adverse effects. In this review, we discuss, using pyrrolizidine alkaloids (PAs) as an example, the hepatotoxicity and tumorigenicity induced by metabolic activation of herbal components and by herb-herb and herb-drug interactions with other herbal ingredients and synthetic drugs. PAs are constitutively produced by plants as the secondary metabolites. There are more than 600 PAs and PA N-oxides identified in over 6000 plants, and more than half of them exhibit hepatotoxicity. Toxic PA-containing plants grow in many geographical regions worldwide, rendering it highly possible that PA-containing plants are the most common poisonous plants affecting livestock and humans. PAs require metabolic activation mediated by cytochrome P450 enzymes to generate reactive pyrrolic metabolites that react with cellular proteins and DNA leading to hepatotoxicity and genotoxicity. PAs can also modulate both phase I and phase II metabolizing enzymes, which may alter the metabolic fate of endogenous and exogenous chemicals. Alteration and/or competition of the metabolizing enzymes by PAs upon the co-administered herbal medicines or drugs can potentially result in serious clinical and toxicological consequences through decreased pharmacological activities or increased toxic effects.
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.
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•Over half (58%) of Chinese retail honeys tested are contaminated with PAs.•Lycopsamine, echimidine, and monocrotaline are main PAs in Chinese honeys tested.•Crotalaria spp., Senecio spp., and Echium plantagineum are the plant origins of PAs.•PAs in Chinese honeys tested present geographically distinct patterns.•About 12% of PA-contaminated Chinese honeys may pose potential health risk.
Pyrrolizidine alkaloids (PAs) are phytotoxins distributed in ∼6000 plant species. PA-contaminated/containing foodstuffs/herbs/supplements pose a potential threat to human health. Various regulatory ...authorities established different PA margins of exposure assuming an equal hepatotoxic potency of structurally diverse PAs, although they exhibit different toxic potencies. Therefore, understanding hepatotoxic potencies of different PAs would facilitate a more appropriate risk assessment of PA exposure. In this study, a zebrafish model, which mimics physiological processes of absorption, distribution, metabolism, and excretion, was selected to evaluate acute hepatotoxic potency of different PAs (7 PAs and 2 PA N-oxides) and explore possible physiological pathways involved in PA-induced hepatotoxicity. After 6 h oral administration, PAs caused distinct structure-dependent hepatotoxicity with a series of biochemical and histological changes in zebrafish. Based on the measured toxicological endpoints, the relative toxic potency order of different PAs was derived as lasiocarpine ∼ retrorsine > monocrotaline > riddelliine > clivorine > heliotrine > retrorsine N-oxide ∼ riddelliine N-oxide≫>platyphyline. Further, compared to control group, different upregulation/downregulation of mRNA expression in PA-treated groups indicated that inflammation, apoptosis, and steatosis were involved in PA-induced hepatotoxicity in zebrafish. These findings demonstrate that zebrafish model is useful for screening and ranking hepatotoxicity of PAs with diverse structures, which would facilitate the more accurate risk assessment of PA exposure.
•PAs produce pyrrole-protein adducts and cause distinct structure-dependent hepatotoxicity in zebrafish.•The zebrafish model is useful for screening and ranking hepatotoxic potency of structurally different PAs.•Inflammation, apoptosis, and steatosis are involved in PA-induced hepatotoxicity in zebrafish.
Pyrrolizidine alkaloids (PAs) widely distribute in plants and can cause hepatic sinusoidal obstruction syndrome (HSOS), which typically presents as a primary sinusoidal endothelial cell damage. It is ...well-recognized that after ingestion, PAs undergo hepatic cytochromes P450 (CYPs)-mediated metabolic activation to generate dehydropyrrolizidine alkaloids (DHPAs), which are hydrolyzed to dehydroretronecine (DHR). DHPAs and DHR are reactive metabolites having same core pyrrole moiety, and can bind proteins to form pyrrole-protein adducts, which are believed as the primary cause for PA-induced HSOS. However, to date, the direct evidences supporting the toxicity of DHPAs and DHR in the liver, in particular in the sinusoidal endothelial cells, are lacking. Using human hepatic sinusoidal endothelial cells (HSEC) and HepG2 (representing hepatic parenchymal cells), cells that lack CYPs activity, this study determined the direct cytotoxicity of dehydromonocrotaline, a representative DHPA, and DHR, but no cytotoxicity of the intact PA (monocrotaline) in both cell lines, confirming that reactive metabolites mediate PA intoxication. Comparing with HepG2, HSEC had significantly lower basal glutathione (GSH) level, and was significantly more susceptible to the reactive metabolites with severer GSH depletion and pyrrole-protein adducts formation. The toxic potency of two reactive metabolites was also compared. DHPA was more reactive than DHR, leading to severer toxicity. In conclusion, our results unambiguously provided the first direct evidence for the critical role of DHPA and DHR in the reactive metabolites-mediated PA-induced hepatotoxicity, which occurs predominantly in HSEC due to severe GSH depletion and the significant formation of pyrrole-protein adducts in HSEC.
•Reactive metabolites (DHPA and DHR) of PA but not intact PA exerted hepatotoxicity.•HSEC were more susceptible to the reactive metabolites compared with HepG2.•Susceptibility of HSEC correlated to GSH depletion and pyrrole-protein adduction.•Comparing the two reactive metabolites, DHPA was more toxic than DHR.
Genotoxicity of pyrrolizidine alkaloids Chen, Tao; Mei, Nan; Fu, Peter P.
Journal of applied toxicology,
April 2010, Letnik:
30, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Pyrrolizidine alkaloids (PAs) are common constituents of many plant species around the world. PA‐containing plants are probably the most common poisonous plants affecting livestock and wildlife. They ...can inflict harm to humans through contaminated food sources, herbal medicines and dietary supplements. Half of the identified PAs are genotoxic and many of them are tumorigenic. The mutagenicity of PAs has been extensively studied in different biological systems. Upon metabolic activation, PAs produce DNA adducts, DNA cross‐linking, DNA breaks, sister chromatid exchange, micronuclei, chromosomal aberrations, gene mutations and chromosome mutations in vivo and in vitro. PAs induced mutations in the cII gene of rat liver and in the p53 and K‐ras genes of mouse liver tumors. It has been suggested that all PAs produce a set of (±)‐6,7‐dihydro‐7‐hydroxy‐1‐hydroxymethyl‐5H‐pyrrolizine‐derived DNA adducts and similar types of gene mutations. The signature types of mutations are G : C → T : A transversion and tandem base substitutions. Overall, PAs are mutagenic in vivo and in vitro and their mutagenicity appears to be responsible for the carcinogenesis of PAs. Published in 2010 by John Wiley & Sons, Ltd.
Pyrrolizidine alkaloids (PAs) from many plants are toxic to livestock, wildlife and human. Many of them are genotoxic and tumorigenic. Upon metabolic activation, PAs produce DNA adducts, DNA cross‐linking, DNA breaks, sister chromatid exchange, micronuclei, chromosomal aberrations, gene mutations, and chromosome mutations invivo and in vitro. The signature types of mutations are G : C → T : A transversion and tandem base substitutions. Overall, PAs are mutagenic and their mutagenicity appears to be responsible for the carcinogenesis of PAs.
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.
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.
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.
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 Papp AtoB values (1.43–16.26) × 10−6 cm/s higher than those of corresponding N-oxides with Papp 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.
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.
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