Mutagenicity of pyrrolizidine alkaloids in the Salmonella/mammalian-microsome test

The chemical structure impairs the intensity of genotoxic effects promoted by 1,2-unsaturated pyrrolizidine alkaloids in vitro

1,2-unsaturated pyrrolizidine alkaloids (PAs) represent a large group of secondary plant metabolites exhibiting hepatotoxic, genotoxic, and carcinogenic properties upon bioactivation. To examine how the degree of esterification affects the genotoxic profile of PA we investigated cytotoxicity, histone H2AX phosphorylation, DNA strand break induction, cell cycle perturbation, micronuclei formation, and aneugenic effects in different cell models. Analysis of cytotoxicity and phosphorylation of histone H2AX was structure- and concentration-dependent: diester-type PAs (except monocrotaline) showed more pronounced effects than monoester-type PAs. Cell cycle analysis identified that diester-type PAs induced a S-phase arrest and a decrease in the occurrence of cells in the G1-phase. The same structure-dependency was observed by flow-cytometric analysis of PA-induced micronuclei in CYP3A4-overexpressing V79 cells. Analysis of centromeres induced by lasiocarpine in the micronuclei by fluorescence in situ hybridization indicated an aneugenic effect in V79_h3A4 cells. Comet assays revealed no significant induction of DNA strand breaks for all investigated PAs. Overall, diester-type PAs induced more pronounced effects than monoester-type PAs. Furthermore, our results indicate aneugenic effects upon exposure towards lasiocarpine in vitro. These data improve our understanding how structural features of PA influence the genotoxic profile. Especially, the monoester-type PAs seem to induce less severe effects than other PAs.

Plasma concentration profiles for hepatotoxic pyrrolizidine alkaloid senkirkine in humans extrapolated from rat data sets using a simplified physiologically based pharmacokinetic model

AIM

The main aim of the current study was to obtain forward dosimetry assessments of pyrrolizidine alkaloid senkirkine plasma and liver concentrations by setting up a human physiologically based pharmacokinetic (PBPK) model based on the limited information available.

BACKGROUND

The risks associated with plant-derived pyrrolizidine alkaloids as natural toxins have been assessed.

OBJECTIVE

The pyrrolizidine alkaloid senkirkine was investigated because it was analyzed in a European transcriptomics study of natural hepatotoxins and in a study of the alkaloidal constituents of traditional Japanese food plants Petasites japonicus. The in silico human plasma and liver concentrations of senkirkine were modeled using doses reported for acute-term toxicity in humans.

METHODS

Using a simplified PBPK model established using rat pharmacokinetic data, forward dosimetry was conducted. Since in vitro rat and human intrinsic hepatic clearances were similar; an allometric scaling approach was applied to rat parameters to create a human PBPK model.

RESULTS

After oral administration of 1.0 mg/kg in rats in vivo, water-soluble senkirkine was absorbed and cleared from plasma to two orders of magnitude below the maximum concentration in 8 h. Human in silico senkirkine plasma concentration curves were generated after virtual daily oral administrations of 3.0 mg/kg senkirkine (the dose involved in an acute fatal hepatotoxicity case). A high concentration of senkirkine in the culture medium caused in vitro hepatotoxicity as evidenced by lactate dehydrogenase leakage from human hepatocyte-like HepaRG cells.

CONCLUSION

Higher virtual concentrations of senkirkine in human liver and plasma than those in rat plasma were estimated using the current rat and human PBPK models. Current simulations suggest that if P. japonicus (a water-soluble pyrrolizidine alkaloid-producing plant) is ingested daily as food, hepatotoxic senkirkine could be continuously present in human plasma and liver.

Chemical Constituents and Biological Properties of Genus Doronicum (Asteraceae)

The genus Doronicum, belonging to tribe Senecioneae (Fam. Asteraceae), is found mainly in the Asia, Europe and North Africa. This genus of plant has always been used in traditional medicinal treatments due to the many biological properties shown such as killing parasitic worms and for relieving constipation, as well as to improve heart health, to alleviate pain and inflammation, to treat insect bites, etc. According to the World Flora the genus Doronicum contains 39 subordinate taxa.^[1-3] The purpose of this article, which covers data published from 1970 to 2021 with more than 110 articles, aims to carry out a complete and critical review of the Doronicum genus, examining traditional uses and reporting the antioxidant, antimicrobial, anti-inflammatory and antitumor activity shown from crude extracts or essential oils, and from single isolated compounds. Furthermore, critical considerations of the published data have been highlighted by comparing them with the results obtained from species of other genus belonging to the Asteraceae family.

In vitro and in vivo genotoxicity assessment of ferric ferrocyanide and potassium-cobalt ferrocyanide

Ferric hexacyanoferrate(II) (Fe₄[Fe(CN)₆]₃), i.e. Prussian blue (PB) has been used for many years to remove from the body the two toxic isotopes of cesium and thallium following irradiation. Recently, potassium cobalt hexacyanoferrate(II) (K₂COFe(CN)₆), which has shown a better efficacy for decontamination, is also being considered for use to enhance the elimination of cesium isotopes. In view to its preclinical and clinical development, in vitro and in vivo GLP-compliant genotoxicity studies were carried out on this product as well as on PB for comparison. Several tests dissecting the main events leading to genotoxicity, i.e. mutagenicity and chromosomal aberrations, both structural and quantitative were implemented. In vitro, no mutagenic effect was observed in the Ames test but both compounds were positive in the mouse lymphoma assay on TK locus and induced clastogenic effects in the in vitro chromosomal aberrations test on human lymphocytes, either in absence or in presence of metabolic activation. K-Co-ferrocyanide was also assayed in vivo in the mouse bone marrow micronucleus assay and PB was assessed for DNA fragmentation in the rodent Comet assay in both glandular stomach and colon. In the in vivo micronucleus mouse bone marrow, K-Co-ferrocyanide did not display any genotoxic activity up to 2000 mg/kg/d (x2) by oral route. In opposite, PB induced a significant increase in DNA fragmentation both in the glandular stomach and in the colon of rat treated 3 times with intake ranging from 2000 to 500 mg/kg. PB should be considered as an in vivo mutagen as well as Potassium cobalt hexacyanoferrate(II) since the in vitro genotoxicity profiles of both ferrocyanides are quite similar. Their use as cesium/ thallium decontamination agents in human should be assessed following a benefit/risk approach to enable a robust decision-making.

Structure-dependent genotoxic potencies of selected pyrrolizidine alkaloids in metabolically competent HepG2 cells

1,2-unsaturated pyrrolizidine alkaloids (PAs) are natural plant constituents comprising more than 600 different structures. A major source of human exposure is thought to be cross-contamination of food, feed and phytomedicines with PA plants. In humans, laboratory and farm animals, certain PAs exert pronounced liver toxicity and can induce malignant liver tumors in rodents. Here, we investigated the cytotoxicity and genotoxicity of eleven PAs belonging to different structural classes. Although all PAs were negative in the fluctuation Ames test in Salmonella, they were cytotoxic and induced micronuclei in human HepG2 hepatoblastoma cells over-expressing human cytochrome P450 3A4. Lasiocarpine and cyclic diesters except monocrotaline were the most potent congeners both in cytotoxicity and micronucleus assays with concentrations below 3 μM inducing a doubling in micronuclei counts. Other open di-esters and all monoesters exhibited weaker or much weaker geno- and cytotoxicity. The findings were in agreement with recently suggested interim Relative Potency (iREP) factors with the exceptions of europine and monocrotaline. A more detailed micronuclei analysis at low concentrations of lasiocarpine, retrorsine or senecionine indicated that pronounced hypolinearity of the concentration–response curves was evident for retrorsine and senecionine but not for lasiocarpine. Our findings show that the genotoxic and cytotoxic potencies of PAs in a human hepatic cell line vary in a structure-dependent manner. Both the low potency of monoesters and the shape of prototype concentration–response relationships warrant a substance- and structure-specific approach in the risk assessment of PAs.

Pyrrolizidine alkaloid-induced alterations of prostanoid synthesis in human endothelial cells

Pyrrolizidine alkaloids (PA) are a group of secondary plant metabolites belonging to the most widely distributed natural toxins. PA intoxication of humans leads to severe liver damage, such as hepatomegaly, hepatic necrosis, fibrosis and cirrhosis. An acute consequence observed after ingestion of high amounts of PA is veno-occlusive disease (VOD) where the hepatic sinusoidal endothelial cells are affected. However, the mechanisms leading to VOD after PA intoxication remain predominantly unknown. Thus, we investigated PA-induced molecular effects on human umbilical vein endothelial cells (HUVEC). We compared the effects of PA with the effects of PA metabolites obtained by in vitro metabolism using liver homogenate (S9 fraction). In vitro-metabolized lasiocarpine and senecionine resulted in significant cytotoxic effects in HUVEC starting at 300 μM. Initial molecular effect screening using a PCR array with genes associated with endothelial cell biology showed PA-induced upregulation of the Fas receptor, which is involved in extrinsic apoptosis, and regulation of a number of interleukins, as well as of different enzymes relevant for prostanoid synthesis. Modulation of prostanoid synthesis was subsequently studied at the mRNA and protein levels and verified by increased release of prostaglandin I₂ as the main prostanoid of endothelial cells. All effects occurred only with in vitro-metabolically activated PA lasiocarpine and senecionine. By contrast, no effect was observed for the PA echimidine, heliotrine, lasiocarpine, senecionine, senkirkine and platyphylline in the absence of an external metabolizing system up to the highest tested concentration of 500 μM. Overall, our results confirm the metabolism-dependent toxification of PA and elucidate the involved pathways. These include induction of inflammatory cytokines and deregulation of the prostanoid synthesis pathway in endothelial cells, linking for the first time PA-dependent changes in prostanoid release to distinct alterations at the mRNA and protein levels of enzymes of prostanoid synthesis.

Interim relative potency factors for the toxicological risk assessment of pyrrolizidine alkaloids in food and herbal medicines

Pyrrolizidine alkaloids (PAs) are among the most potent natural toxins occurring in a broad spectrum of plant species from various families. Recently, findings of considerable contamination of teas/herbal infusions prepared from non-PA plants have been reported. These are obviously due to cross-contamination with minor amounts of PA plants and can affect both food and herbal medicines. Another source of human exposure is honey collected from PA plants. These findings illustrate the requirement for a comprehensive risk assessment of PAs, hampered by the enormous number of different PA congeners occurring in nature. Up to now, risk assessment is based on the carcinogenicity of certain PAs after chronic application to rats using the sum of detected PAs as dose metric. Because of the well-documented large structure-dependent differences between sub-groups of PA congeners with respect to their genotoxicity and (cyto)toxicity, however, this procedure is inadequate. Here we provide an overview of recent attempts to assess the risk of PA exposure and the available literature on the toxic effects and potencies of different congeners. Based on these considerations, we have derived interim Relative Potency (REP) factors for a number of abundant PAs suggesting a factor of 1.0 for cyclic di-esters and open-chain di-esters with 7S configuration, of 0.3 for mono-esters with 7S configuration, of 0.1 for open-chain di-esters with 7R configuration and of 0.01 for mono-esters with 7R configuration. For N-oxides we suggest to apply the REP factor of the corresponding PA. We are confident that the use of these values can provide a more scientific basis for PA risk assessment until a more detailed experimental analysis of the potencies of all relevant congeners can be carried out.

Are effects of common ragwort in the Ames test caused by pyrrolizidine alkaloids?

It has previously been demonstrated by others that acetone extracts of Senecio jacobaea (syn. Jacobaea vulgaris, common or tansy ragwort) test positive in the Salmonella/microsome mutagenicity test (Ames test). Pyrrolizidine alkaloids (PAs) are thought to be responsible for these mutagenic effects. However, it was also observed that the major PA present in common ragwort, jacobine, produced a negative response (with and without the addition of rat liver S9) in Salmonella test strains TA98, TA100, TA1535 and TA1537. To investigate which compounds in the plant extracts were responsible for the positive outcome, the present study investigated the contents and mutagenic effects of methanol and acetone extracts prepared from dried ground S. jacobaea and Senecio inaequidens (narrow-leafed ragwort). Subsequently, a fractionation approach was set up in combination with LC-MS/MS analysis of the fractions. It was shown that the positive Ames test outcomes of S. jacobaea extracts are unlikely to be caused by PAs, but rather by the flavonoid quercetin. This study also demonstrates the importance of identifying compounds responsible for positive test results in bioassays.

Comparison of in vitro micronucleus and gene mutation assay results for p53-competent versus p53-deficient human lymphoblastoid cells

The high frequency of false or irrelevant positive results in in vitro mammalian cell genotoxicity tests is a critical concern for regulators. Here, we tested whether such results may be due to the mammalian cells used in the tests being deficient in p53, which is involved in the maintenance of genomic stability. We compared the in vitro responses of two human lymphoblastoid cell lines derived from the same progenitor cell-p53-competent (TK6) and p53-deficient (WTK-1) cells-in a micronucleus (MN) test and a thymidine kinase gene (TK) mutation assay. We tested 14 chemicals including three mutagens and 11 clastogens and spindle poisons. The three mutagens evoked clear positive responses in both assays in both cell lines. The responses to the clastogens and spindle poisons, on the other hand, depended on the assay endpoint and/or the cell line. Most of clastogens and spindle poisons were positive in the MN test in both cell lines. In the TK mutation assay, on the other hand, WTK-1 cells but not TK6 cells detected spindle poisons, which may have been due to the disturbance of the spindle checkpoint and lack of apoptosis in the p53-deficient cells. Some chemicals that induced chromosome aberrations in rodent cells were negative in both TK6 and WTK-1 cells, indicating that a species-specific factor rather than p53 status was associated with the response. In conclusion, the p53 status did not seriously influence the MN test results but it did influence the TK mutation assay results.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Metabolism, genotoxicity, and carcinogenicity of comfrey

Comfrey has been consumed by humans as a vegetable and a tea and used as an herbal medicine for more than 2000 years. Comfrey, however, produces hepatotoxicity in livestock and humans and carcinogenicity in experimental animals. Comfrey contains as many as 14 pyrrolizidine alkaloids (PA), including 7-acetylintermedine, 7-acetyllycopsamine, echimidine, intermedine, lasiocarpine, lycopsamine, myoscorpine, symlandine, symphytine, and symviridine. The mechanisms underlying comfrey-induced genotoxicity and carcinogenicity are still not fully understood. The available evidence suggests that the active metabolites of PA in comfrey interact with DNA in liver endothelial cells and hepatocytes, resulting in DNA damage, mutation induction, and cancer development. Genotoxicities attributed to comfrey and riddelliine (a representative genotoxic PA and a proven rodent mutagen and carcinogen) are discussed in this review. Both of these compounds induced similar profiles of 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-derived DNA adducts and similar mutation spectra. Further, the two agents share common mechanisms of drug metabolism and carcinogenesis. Overall, comfrey is mutagenic in liver, and PA contained in comfrey appear to be responsible for comfrey-induced toxicity and tumor induction.

Genotoxicity of pyrrolizidine alkaloids

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.

Protection of S-adenosyl methionine against the toxicity of clivorine on hepatocytes

In this study, we investigated the protective effects of S-adenosyl-l-methionine (SAM), which is a precursor of cellular reduced glutathione (GSH), against the hepatotoxicity of pyrrolizidine alkaloid clivorine. MTT assay showed that SAM (5μM) prevented the cytotoxicity of clivorine on human normal liver L-02 cells. DNA fragmentation assay showed that SAM (5μM) improved clivorine-induced L-02 cell apoptosis, and the results of Western blot showed that SAM (5μM) decreased clivorine-induced caspase-3 activation. Cellular GSH analysis showed that when L-02 cells were exposed to different concentrations (0, 3, 10, 30, 50 and 100μM) of clivorine for 48h, cellular GSH was decreased in a concentration-dependent manner, while SAM (5μM) enhanced 50μM clivorine decreased cellular GSH. Further MTT assay showed that 5mM GSH and 5mM N-acetyl-l-cysteine (NAC) both had protective effects against clivorine-induced hepatotoxicity. Our results suggest that SAM has protective effects against the hepatotoxicity of clivorine possibly by enhancing cellular GSH level and increasing cellular defensive ability against clivorine-induced cytotoxicity.

The toxic effect of pyrrolizidine alkaloid clivorine on the human embryonic kidney 293 cells and its primary mechanism

Clivorine is an otonecine-type pyrrolizidine alkaloid (PA) isolated from the Chinese medicinal plant Ligularia hodgsonii Hook., and our previous reports have shown its toxicity on human normal liver L-02 cells. It is generally believed that biotransformation of PAs to its metabolites is required for their toxicity; thus, there is nearly no report about the toxicity of clivorine on other non-hepatic cells in vitro. The aim of this study is to observe the toxicity of clivorine on the non-hepatic human embryonic kidney 293 (HEK293) cell that is of epithelial origin, and its primary mechanism. Our results showed that clivorine significantly reduced HEK293 cell viability, but there was no detectable apoptotic DNA ladder and cleaved fragments of caspase-3 and caspase-9 in clivorine-treated cells, which indicates the toxicity of clivorine is not due to inducing apoptosis. The results of western blot showed that clivorine induced sustained p38, c-Jun N-terminal kinase (JNK) and extracellular signal-related kinases (ERK1/2) phosphorylation in a concentration- and time-dependent manner, and the JNK inhibitor SP600125 significantly augmented the toxicity of clivorine. Our results suggest that clivorine itself has direct toxicity on HEK293 cells, and phosphorylated JNK may play some role in counteracting the toxicity of clivorine on HEK293 cells.

Deacetylclivorine: a gender-selective metabolite of clivorine formed in female Sprague-Dawley rat liver microsomes

Clivorine, a naturally occurring pyrrolizidine alkaloid, causes liver toxicity via its metabolic activation to generate toxic metabolite (pyrrolic ester). Female Sprague-Dawley (SD) rats are reported to be less susceptible to clivorine intoxication than male SD rats. However, the biochemical mechanism causing such gender difference is largely unknown. The present study investigated hepatic microsomal metabolism of clivorine in female rats to delineate the mechanism of the gender difference. Two pathways, which directly metabolize clivorine, were observed. First, the metabolic activation to produce the toxic pyrrolic ester followed by formations of bound pyrroles, dehydroretronecine, 7-glutathionyldehydroretronecine, and clivoric acid were found in female rats, and CYP3A1/2 isozymes were identified to catalyze the metabolic activation. Compared with male rats ( approximately 21%), the metabolic activation in female rats was significantly lower ( approximately 4%) possibly because of significantly lower CYP3A1/2 levels expressed in female rats. Second, a direct hydrolysis to generate the novel female rat-specific metabolite deacetylclivorine was shown as the predominant pathway ( approximately 16% clivorine metabolism) in female rat liver microsomes and was determined to be mediated by microsomal hydrolase A. Furthermore, when the metabolic activation was completely inhibited by ketoconazole, the amount of deacetylclivorine formed in a 1-h incubation significantly increased from 19.44 +/- 3.00 to 54.87 +/- 9.30 nmol/mg protein, suggesting that the two pathways compete with each other. Therefore, the lower susceptibility of female SD rats to clivorine intoxication is suggested to be caused by the significantly higher extent of the direct hydrolysis and a lower degree of the metabolic activation.

Evaluation of the ability of a battery of three in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens I. Sensitivity, specificity and relative predictivity

The performance of a battery of three of the most commonly used in vitro genotoxicity tests--Ames+mouse lymphoma assay (MLA)+in vitro micronucleus (MN) or chromosomal aberrations (CA) test--has been evaluated for its ability to discriminate rodent carcinogens and non-carcinogens, from a large database of over 700 chemicals compiled from the CPDB ("Gold"), NTP, IARC and other publications. We re-evaluated many (113 MLA and 30 CA) previously published genotoxicity results in order to categorise the performance of these assays using the response categories we established. The sensitivity of the three-test battery was high. Of the 553 carcinogens for which there were valid genotoxicity data, 93% of the rodent carcinogens evaluated in at least one assay gave positive results in at least one of the three tests. Combinations of two and three test systems had greater sensitivity than individual tests resulting in sensitivities of around 90% or more, depending on test combination. Only 19 carcinogens (out of 206 tested in all three tests, considering CA and MN as alternatives) gave consistently negative results in a full three-test battery. Most were either carcinogenic via a non-genotoxic mechanism (liver enzyme inducers, peroxisome proliferators, hormonal carcinogens) considered not necessarily relevant for humans, or were extremely weak (presumed) genotoxic carcinogens (e.g. N-nitrosodiphenylamine). Two carcinogens (5-chloro-o-toluidine, 1,1,2,2-tetrachloroethane) may have a genotoxic element to their carcinogenicity and may have been expected to produce positive results somewhere in the battery. We identified 183 chemicals that were non-carcinogenic after testing in both male and female rats and mice. There were genotoxicity data on 177 of these. The specificity of the Ames test was reasonable (73.9%), but all mammalian cell tests had very low specificity (i.e. below 45%), and this declined to extremely low levels in combinations of two and three test systems. When all three tests were performed, 75-95% of non-carcinogens gave positive (i.e. false positive) results in at least one test in the battery. The extremely low specificity highlights the importance of understanding the mechanism by which genotoxicity may be induced (whether it is relevant for the whole animal or human) and using weight of evidence approaches to assess the carcinogenic risk from a positive genotoxicity signal. It also highlights deficiencies in the current prediction from and understanding of such in vitro results for the in vivo situation. It may even signal the need for either a reassessment of the conditions and criteria for positive results (cytotoxicity, solubility, etc.) or the development and use of a completely new set of in vitro tests (e.g. mutation in transgenic cell lines, systems with inherent metabolic activity avoiding the use of S9, measurement of genetic changes in more cancer-relevant genes or hotspots of genes, etc.). It was very difficult to assess the performance of the in vitro MN test, particularly in combination with other assays, because the published database for this assay is relatively small at this time. The specificity values for the in vitro MN assay may improve if data from a larger proportion of the known non-carcinogens becomes available, and a larger published database of results with the MN assay is urgently needed if this test is to be appreciated for regulatory use. However, specificity levels of <50% will still be unacceptable. Despite these issues, by adopting a relative predictivity (RP) measure (ratio of real:false results), it was possible to establish that positive results in all three tests indicate the chemical is greater than three times more likely to be a rodent carcinogen than a non-carcinogen. Likewise, negative results in all three tests indicate the chemical is greater than two times more likely to be a rodent non-carcinogen than a carcinogen. This RP measure is considered a useful tool for industry to assess the likelihood of a chemical possessing carcinogenic potential from batteries of positive or negative results.

Pyrrolizidine alkaloid clivorine induces apoptosis in human normal liver L-02 cells and reduces the expression of p53 protein

Clivorine, a pyrrolizidine alkaloid, is a potent toxic compound extracted from a Chinese medicinal plant Ligularia hodgsonii Hook. We have previously shown that clivorine inhibited human normal liver L-02 cells proliferation and induced p38 mitogen-activated protein kinase phosphorylation. The aim of this study is to further observe the mechanism of clivorine-inhibited L-02 cells growth. In this paper we show here that clivorine can induce apoptosis in L-02 cells, reduce the expression of p53 protein but has no effect on the expression of Bcl-2 protein, and clivorine also induces the cleavage of the poly(ADP-ribose) polymerase in L-02 cells. Our results suggest that the anti-proliferative function of clivorine in L-02 cells may be due to its inducing cell apoptosis, and clivorine-induced cell apoptosis is independent of p53 protein.

Pyrrolizidine Alkaloids—Genotoxicity, Metabolism Enzymes, Metabolic Activation, and Mechanisms

Pyrrolizidine alkaloid-containing plants are widely distributed in the world and are probably the most common poisonous plants affecting livestock, wildlife, and humans. Because of their abundance and potent toxicities, the mechanisms by which pyrrolizidine alkaloids induce genotoxicities, particularly carcinogenicity, were extensively studied for several decades but not exclusively elucidated until recently. To date, the pyrrolizidine alkaloid-induced genotoxicities were revealed to be elicited by the hepatic metabolism of these naturally occurring toxins. In this review, we present updated information on the metabolism, metabolizing enzymes, and the mechanisms by which pyrrolizidine alkaloids exert genotoxicity and tumorigenicity.

The effect of Senecio latifolius a plant used as a South African traditional medicine, on a human hepatoma cell line

A number of traditional remedies used in South Africa contain pyrrolizidine alkaloids, some of which are hepatotoxic. We investigated the effect on human HuH-7 cells of Senecio latifolius DC., a plant that is a component of some traditional remedies and which is known to contain toxic pyrrolizidine alkaloids. Cells were also treated with extracts of a standard pyrrolizidine, retrorsine. The changes in the gross morphology of the cells were studied using light microscopy after haematoxylin and eosin staining. The cytoskeleton was investigated using fluorescence-labelled anti-beta-tubulin antibody and the nuclear organisation was studied using fluorescence-labelled antinuclear antibodies. The plant extracts gave rise to dose-dependent gross morphological changes. At high doses, we observed necrosis and at lower doses, destruction of the cytoskeleton, nuclear fragmentation and apoptosis. Doses of less than the equivalent of 330 ng/ml retrorsine led to multinucleated cells with failure in spindle formation and clumping of nuclear chromatin. This latter finding suggests that chronic low-dose treatment with such traditional remedies could give rise to teratogenic and/or carcinogenic effects.

Three glycoproteins with antimutagenic activity identified in Lactobacillus plantarum KLAB21

Antimutagenic substances were purified from a culture supernatant of Lactobacillus plantarum KLAB21 cells isolated from kimchi, a Korean traditional fermented vegetable, and their characteristics were investigated. The antimutagenic substances were separated into two fractions by DEAE-cellulose ion-exchange column chromatography, which were designated the R1 and R2 fractions. The R1 fraction was then divided into two fractions again by Sephadex G200 gel filtration chromatography, and the fractions were designated R1-1 and R1-2. All three fractions were further purified using a Sepharose CL-6B gel filtration column. All the purified fractions were successfully stained with fuchsin as well as Coomassie brilliant blue, suggesting that they are glycoproteins. The purified fractions were confirmed to possess antimutagenic activity against N-methyl-N'-nitro-N-nitrosoguanidine on Salmonella enterica serovar Typhimurium TA100 cells. Their molecular masses were determined to be 16 (R1-1), 11 (R1-2), and 14 (R2) kDa on the Sepharose CL-6B column. Total sugar contents were 8.4% (R1-1), 7.3% (R1-2), and 9.4% (R2). The amino acid compositions of the fractions were different from each other; the major amino acids were glutamic acid (21.5%) and phenylalanine (17.1%) in the R1-1 fraction and glycine (41.3%) in the R1-2 fraction, but valine (31%) and phenylalanine (22.6%) were the major amino acids in the R2 fraction.

Should we be concerned about herbal remedies

During the latter part of this century the practice of herbalism has become mainstream throughout the world. This is due in part to the recognition of the value of traditional medical systems, particularly of Asian origin, and the identification of medicinal plants from indigenous pharmacopeias that have been shown to have significant healing power, either in their natural state or as the source of new pharmaceuticals. Generally these formulations are considered moderate in efficacy and thus less toxic than most pharmaceutical agents. In the Western world, in particular, the developing concept that 'natural' is better than 'chemical' or 'synthetic' has led to the evolution of Neo-Western herbalism that is the basis of an ever expanding industry. In the US, often guised as food, or food supplements, known as nutriceuticals, these formulations are readily available for those that wish to self-medicate. Within this system, in particular, are plants that lack ethnomedical verification of efficacy or safety. Unfortunately there is no universal regulatory system in place that insures that any of these plant remedies are what they say they are, do what is claimed, or most importantly are safe. Data will be presented in this context, outlining how adulteration, inappropriate formulation, or lack of understanding of plant and drug interactions have led to adverse reactions that are sometimes life-threatening or lethal.

Characterization of two structural forms of otonecine-type pyrrolizidine alkaloids from Ligularia hodgsonii by NMR spectroscopy

Clivorine (1) and ligularine (2), two hepatotoxic otonecine-type pyrrolizidine alkaloids isolated from Ligularia hodgsonii, an antitussive traditional Chinese medicine, were investigated in CDCl(3) and D(2)O by various NMR techniques to delineate why this type of alkaloid displays uncharacteristic solubility properties by dissolving in both nonpolar organic and aqueous solutions. The results demonstrated that both alkaloids exist in a non-ionized form in CDCl(3), but in an ionized form in D(2)O, suggesting that this unique dual solubility may play a role in the intoxication resultant from consumption of water extracts of herbs, including herbal teas, containing otonecine-type pyrrolizidine alkaloids.

Nutrition and dietary carcinogens

Three major factors for human carcinogenesis are (i) cigarette smoking, (ii) infection and inflammation and (iii) nutrition and dietary factors. Nutrition and dietary factors include two categories, namely genotoxic agents and constituents including tumor promotion-associated phenomena. This article first describes the genotoxic agents as microcomponents. These are mutagens/carcinogens in cooked food, fungal products, plant and mushroom substance, and nitrite-related materials, polycyclic aromatic hydrocarbons and oxidative agents. Emphasis has been given to heterocyclic amines (HCAs) to which humans are continuously exposed in an ordinary lifestyle. HCAs in food are mainly produced from creatin(in)e, sugar and from amino acids in meat (upon heating). They are imidazoquinoline and imidazoquinoxaline derivatives and phenylimidazopyridine. HCAs are pluripotent in producing cancers in various organs including breast, colon and prostate. Discussion is also given to plant flavonoids which are mutagenic but not carcinogenic. As a macrocomponent, overintake of total calories, fat and sodium chloride is discussed from the viewpoint of the increase of genetic alterations in tissues and of tumor promotion-associated issues. Studies of nutrition and dietary condition will eventually lead us to cancer prevention, namely delay of onset of cancer to the late phase of human life, which is called 'natural-end cancer' (Tenju-gann).

SOS induction of selected naturally occurring substances in Escherichia coli (SOS chromotest)

Naturally occurring substances were tested for genotoxicity using a modified laboratory protocol of the Escherichia coli PQ37 genotoxicity assay (SOS chromotest) in the presence and in the absence of an exogenous metabolizing system from rat liver S9-mix. Aristolochic acid I, II, the plant extract aristolochic acid and psoralene were genotoxic; cycasine, emodine, monocrotaline and retrorsine were classified as marginal genotoxic in the SOS chromotest in the absence of S9-mix. In the presence of an exogenous metabolizing system from rat liver S9-mix aristolochic acid I, the plant extract, beta-asarone, cycasin, monocrotaline, psoralen and retrorsine showed genotoxic effects; aristolochic acid II marginal genotoxic effects. Arecoline, benzyl acetate, coumarin, isatidine dihydrate, reserpine, safrole, sanguinarine chloride, senecionine, senkirkine, tannin and thiourea revealed no genotoxicity in the SOS chromotest either in the presence or in the absence of an exogenous metabolizing system from rat liver S9-mix. For 17 of 20 compounds, the results obtained in the SOS chromotest could be compared to those obtained in the Ames test. It was found that 12 (70.6%) of these compounds give similar responses in both tests (6 positive and 6 negative responses). The present investigation and those reported earlier, the SOS chromotest, using E. coli PQ37, was able to detect correctly most of the Salmonella mutagens and non-mutagens.

Evaluation of the rat bone marrow and peripheral blood micronucleus test using monocrotaline

The present study was performed as part of the 9th collaborative trial of the CSGMT (Collaborative Study Group for the Micronucleus Test) to evaluate the performance of the rat micronucleus test in detection of clastogenic agents. Temporary fluorescent staining using acridine orange (AO) was compared with permanent staining using a modified Feulgen technique for bone marrow and Giemsa for blood. The Feulgen and AO methods were both found to be suitable for demonstrating micronucleus induction in rat bone marrow following oral administration of monocrotaline and cyclophosphamide. Induction of micronuclei was also shown using rat blood. The simple AO supravital method described produced uniformly stained preparations which were extremely easy to assess and interpret. The AO method was preferred to Giemsa for analysis of blood, but it was essential to analyse samples within 4 days of collection to avoid excessive deterioration of cells. Because of the relatively low number of micronucleated cells found in rat blood, it is suggested that examination of a larger number of cells may be necessary to achieve a similar sensitivity to the bone marrow assay for routine screening.

Risks associated with consumption of herbal teas

Plants have been used for medicinal purposes for centuries. Health-oriented individuals are turning to herbal teas as alternatives to caffeinated beverages such as coffee, tea, and cocoa and for low-caloric supplements. The popularity of herbal tea consumption has increased significantly during the past two decades in the U.S. Hundreds of different teas made up of varied mixtures of roots, leaves, seeds, barks, or other parts of shrubs, vines, or trees are sold in health food stores. Although chemists have been characterizing toxic plant constituents for over 100 years, toxicological studies of herbal teas have been limited and, therefore, the safety of many of these products is unknown. Plants synthesize secondary metabolites that are not essential in the production of energy and whose role may be in the defense mechanisms as plant toxins to their interactions with other plants, herbivores, and parasites. Pyrrolizidine alkaloids (PAs) were among the first naturally occurring carcinogens identified in plant products, and their presence in herbal teas is a matter of public health significance. Some herbal tea mixtures and single-ingredient herbal teas have been analyzed for toxic/mutagenic potential by bioassay and chromatographic techniques. Numerous human and animal intoxications have been associated with naturally occurring components, including pyrrolizidine alkaloids, tannins, and safrole. Thus, the prevention of human exposure to carcinogens or mutagens present in herbal tea mixture extracts is crucial. Preparation of infusion drinks prepared from plants appears to concentrate biologically active compounds and is a major source of PA poisoning. The quantity and consumption over a long period of time is of major concern. It is recommended that widespread consumption of herbal infusions should be minimized until data on the levels and varieties of carcinogens, mutagens, and toxicants are made available.

Micronuclei in mice treated with monocrotaline with and without phenobarbital pretreatment

Monocrotaline is a very potent toxin, producing significant effects of pneumotoxicity, hepatotoxicity, and teratogenicity, as well as carcinogenicity. In addition, the compound has been clearly shown to be mutagenic after metabolic activation. The goal of the experiments reported here was to confirm the reported clastogenesis induced by this agent in vivo and to evaluate the impact of modulation of metabolic activity by phenobarbital, a potent P-450 inducer (both Phase I and Phase II enzymes). The method used in addressing this problem relied on a new technique for monitoring clastogenesis in vivo, i.e., the acridine orange micronucleus assay method originally exploited by Hayashi et al. [1990]. The result of our experiments confirmed monocrotaline to be an effective clastogen in vivo, using the acridine orange method of assessment. The peak in induction of micronuclei occurred on the second day following intraperitoneal administration of the drug. Administration of phenobarbital prior to monocrotaline did appear to modulate the micronucleus induction. At 30 mg/kg bw monocrotaline, the pretreatment with phenobarbital appears to increase the intensity of monocrotaline clastogenesis, while the effect at higher doses (60 and 125 mg/kg bw) is a reduction in potency, presumably reflecting increased importance of Phase II metabolism for monocrotaline at these doses. Thus the study reported here confirms the potent in vivo clastogenesis of monocrotaline, and provides evidence for a dose-related shift in mechanism for the phenomenon.

Recommendations for the performance of bacterial mutation assays

At the International Workshop on the Standardisation of Genotoxicity Test Procedures, in Melbourne (27-28 February 1993), the current international guidelines for the correct conduct of bacterial mutation assays were considered, and the major differences between them were examined. An attempt was made to construct a scientifically based, internationally harmonized protocol. The main points of agreement were as follows. The consensus opinion was that there are currently insufficient data to justify a preference for either the preincubation or plate-incorporation methodologies as the initial test. Whichever method is used there was consensus agreement that the bacterial test battery should consist of S. typhimurium TA1537, TA1535, TA98 and TA100. There was also consensus that the 3 strains TA97a, TA97 and TA1537 could be used interchangeably. Although it was not possible to achieve a consensus, the majority of the working group members agreed that strains for the detection of mutagens acting specifically on AT base pairs should be routinely included within the test battery. These strains may be S. typhimurium TA102 or E. coli WP2 strains (WP2 pKM101 and WP2 uvrA or WP2 uvrA pkM101). With regard to study design it was universally agreed that 5 doses of test compound should be used in each experiment, and a majority agreement was obtained for 3 plates per dose. The use of 2 plates per dose is acceptable ONLY if the experiment is repeated. It is recommended that the negative controls may consist of solvent control alone provided that historical data are available to demonstrate lack of effect of the solvent in question. Positive control compounds should be included in all experiments, although the nature of these control compounds need not be specified in the guidelines. There was consensus agreement that for non-toxic freely soluble test agents, an upper limit of 5 mg/plate should be tested (5 microliters per plate for liquids). For insoluble or toxic compounds, the recommendations were the same as those for other in vitro tests (see appropriate paper). A consensus agreement was reached on the need to carry out further tests if equivocal results are obtained in the initial test, although it was generally agreed that the design of the repeat study should be left flexible. As there are little or no data to support the use of an exact repeat assay, a majority of the group recommended that negative results in the first test should be further investigated by either conducting a modified repeat (e.g. S9 titration) or by conducting the alternative methodology.(ABSTRACT TRUNCATED AT 400 WORDS)

Clastogenic effect of extracts obtained from Crotalaria retusa L. and Crotalaria mucronata Desv. on mouse bone marrow cells

This work has evaluated the clastogenicity of six extracts (tea and aqueous extract of leaves, tea, aqueous and methanolic extracts of dried fruit, and tea of unripe fruit) obtained from Crotalaria retusa L. and three extracts (tea and methanolic extract of dried fruit, and tea of unripe fruit) obtained from Crotalaria mucronata Desv. The extracts were injected intraperitoneally into mice, and the animals were killed 24 h after treatment for preparation of bone marrow cells. The extracts obtained from fruits of Crotalaria retusa were found to cause a dose-dependent increase in the frequency of chromosomal aberrations in mice. On the other hand, no statistically significant increase in the frequency of aberrant cells was observed for the animals treated with leaf extracts obtained from Crotalaria retusa and with extracts from fruits of Crotalaria mucronata. The possibility that the pyrrolizidine alkaloid, monocrotaline, present in Crotalaria retusa exerts a clastogenic effect on mouse bone marrow cells is discussed. Our conclusion is based on studies using intraperitoneal treatments. Effects of oral exposure to extracts of Crotalaria retusa are unknown.

Multistep carcinogenesis: a 1992 perspective

Cancer is the leading cause of death in Japan. Recent changes in cancer incidence patterns may reflect the trend toward a more Western diet and life-style. Among the dietary factors that contribute to carcinogenesis are the heterocyclic amines, a group of mutagenic compounds present in cooked meat and fish. Carcinogenesis is a multistep process in which cells accumulate multiple genetic alterations as they progress to a more malignant phenotype. Recognition of the growing number of interacting factors that contribute to carcinogenesis may force reconsideration of current methods of risk assessment.

Structure/activity relationships of the genotoxic potencies of sixteen pyrrolizidine alkaloids assayed for the induction of somatic mutation and recombination in wing cells of Drosophila melanogaster

Sixteen pyrrolizidine alkaloids (PAs) were examined for their genotoxic potency in the wing spot test of Drosophila melanogaster following oral application. This in vivo assay tests for the induction of somatic mutation and mitotic recombination in cells of the developing wing primordia. All PAs tested except the C9-monoester supinine were clearly genotoxic. Depending on their chemical structure, however, genotoxicity of the PAs varied widely in a range encompassing about three orders of magnitude. In general, macrocyclic diester-type PAs were the most and 7-hydroxy C9-monoester types the least genotoxic representatives studied, while open diesters were intermediate in this respect. Stereoisomeric PAs mostly showed similar, but sometimes also clearly unequal genotoxicity. An increasing number of hydroxy groups in the PA molecule seemed to reduce its genotoxic potency. With respect to the structure/activity relationships, there appears to be a good correlation between hepatotoxicity of PAs in experimental rodents and genotoxicity in the wing spot test of Drosophila. This suggests that PAs are bioactivated along similar pathways in the mammalian liver and in the somatic cells of Drosophila. The genotoxic potential of PAs in the Drosophila wing spot test and their carcinogenic potential in mammals also seem correlated, although the information in the literature on carcinogenicity of the non-macrocyclic PAs with moderate to low genotoxic potency is concededly limited. Comparisons with other genotoxicity tests suggest that the wing spot test is particularly suitable for genotoxins like PAs, on the one hand because of the versatile metabolic bioactivation system of Drosophila and on the other hand also because of its excellent sensitivity to the crosslinking agents among the genotoxins.

Mutagenicity of pyrrolizidine alkaloids in the Salmonella typhimurium/mammalian microsome system

The mutagenicity of a series of pyrrolizidine alkaloids, and of extracts from several Italian Senecio species containing pyrrolizidine alkaloids, including S. inaequidens, S. fuchsii and S. cacaliaster, were tested using the Salmonella typhimurium/mammalian microsome system. Retrorsine, senecivernine, seneciphylline and the Senecio extracts showed a weakly mutagenic activity.

DNA cross-linking in mammalian cells by pyrrolizidine alkaloids: structure-activity relationships

Pyrrolizidine alkaloids (PAs) are common constituents of many species of flowering plants which possess carcinogenic as well as anticarcinogenic activity in vivo. Pyrrolizidine alkaloids are genotoxic in various short-term assays. The mechanisms by which these compounds exert these effects is still unclear. In this study, we characterized the ability of eight bifunctional PAs, with differing stereochemistry and functional groups, to cross-link cellular DNA in cultured bovine kidney epithelial cells. PAs representative of three major structural classes, the macrocycles (seneciphylline, riddelline, retrorsine, senecionine, monocrotaline), the open diesters (heliosupine, latifoline), and pyrrolizidine base (retronecine) were cultured for 2 hr with cells and an external metabolizing system. Every PA induced DNA cross-links which consisted primarily of proteinase-sensitive cross-links (DPC), but also to a smaller extent, DNA interstrand cross-links (ISC). None of the PAs induced detectable amounts of DNA single-strand breaks. The PAs which produced DPC and/or ISC (ranked from most potent to least) were: seneciphylline (DPC greater than ISC); riddelline (DPC greater than ISC); retrorsine (DPC greater than ISC); senecionine (DPC greater than ISC); heliosupine (DPC greater than ISC); monocrotaline (ISC = DPC); latifoline (DPC greater than ISC); and retronecine (ISC greater than DPC). Although the PAs induced DNA cross-linking to varying degrees, cell viabilities for all treatment groups were greater than 90% as determined by trypan blue dye exclusion. Since the cross-linking ability of these PAs paralleled their ability to inhibit colony formation, cross-link formation may be involved in the biological activity of these compounds. Two structural determinants of biological activity appear to be the presence of both a macrocyclic necic acid ester and an alpha,beta-unsaturated ester function since the cross-linking ability of seneciphylline, riddelline, retrorsine, and senecionine far exceeded that of monocrotaline, heliosupine, latifoline, and retronecine. In addition, the stereochemical orientation of the ester linkage was found to have no effect on biological activity.

The effect of the pyrrolizidine alkaloid integerrimine on the chromosomes of mouse bone marrow cells

In an investigation of the action of integerrimine on chromosomes, the bone marrow was taken as target organ. Male and female mice of the C57Bl/6 strain received a single acute dose of this pyrrolizidine alkaloid, in 2 concentrations: 18.75 and 37.50 mg/kg. Bone marrow cells were collected 6, 12 and 24 h after treatment. The analysis of metaphasic chromosomes demonstrated that chromosomal damage occurs, correlated with drug concentration. The greatest frequency of chromosomal aberrations was detected 12 h after treatment.

Mutagenicity of isoquinoline alkaloids, especially of the aporphine type

The mutagenicity of 44 isoquinoline alkaloids was tested in Salmonella typhimurium TA100 and TA98 in the presence or absence of S9 mix. The alkaloids tested included compounds from the isoquinoline, benzylisoquinoline, bisbenzylisoquinoline, monoterpene isoquinoline, berberine, morphinane, hasubanan, benzo[c]phenanthridine and aporphine groups. Among the alkaloids tested, liriodenine was the most potent mutagen for TA100 and roemerine was the most potent for TA98. A clear structure-mutagenicity relationship was observed in a series of aporphine alkaloids (aporphine, dehydroaporphine, 7-oxoaporphine and 4,5-dioxoaporphine), and 10,11-non-substituted aporphines were suggested to exert their mutagenicity through metabolic activation of the 10,11 positions, possibly as the 10,11-epoxides.

Mutagenic activity of promutagens in plants: indirect evidence of their activation

This review summarizes data concerning mutagenic activity of promutagens in various plant in vivo assays. These data are compared with the present knowledge about the metabolism of xenobiotics and activation of promutagens in plants obtained by biochemical studies and by the separation of the activation process from the genetic endpoints assayed for the mutagenicity. The article documents a differential response of plant species in the endogenous transforming of various classes of promutagens into mutagens. Attention is devoted to the following types of promutagens: nitrosamines, polycyclic aromatic hydrocarbons and aromatic amines, aflatoxins, pyrrolizidine alkaloids, diallate, styrene, vinylchloride, ethanol, cycasin, nitrofurans, sodium azide, s-triazine herbicides, 1,2-dibromoethane and maleic hydrazide.

Mutagenic activities of dictamnine and gamma-fagarine from dictamni radicis cortex (Rutaceae)

A methanol extract of Dictamni Radicis Cortex exhibited a mutagenic effect on Salmonella typhimurium TA100 and TA98 with S9 mix. Two mutagenic compounds in Dictamni Radicis Cortex were isolated on a Sephadex LH 20 column and silica gel column chromatography and by preparative TLC. These were identified as dictamnine and gamma-fagarine by UV, EI-Mass, 1H-NMR. Dictamnine and gamma-fagarine were mutagenic in strain TA100 and TA98 with S9 mix. The dose-response curves were linear in the range 10-40 micrograms. Dictamnine and gamma-fagarine had specific activities (His+/microgram) of about 50-70 revertant colonies in strain TA100, while in strain TA98 there were about 30-50 revertant colonies.