Obesity and hepatosteatosis in mice with enhanced oxidative DNA damage processing in mitochondria.

Mitochondria play critical roles in oxidative phosphorylation and energy metabolism. Increasing evidence supports that mitochondrial DNA (mtDNA) damage and dysfunction play vital roles in the development of many mitochondria-related diseases, such as obesity, diabetes mellitus, infertility, neurodegenerative disorders, and malignant tumors in humans. Human 8-oxoguanine-DNA glycosylase 1 (hOGG1) transgenic (TG) mice were produced by nuclear microinjection. Transgene integration was analyzed by PCR. Transgene expression was measured by RT-PCR and Western blot analysis. Mitochondrial DNA damage was analyzed by mutational analyses and measurement of mtDNA copy number. Total fat content was measured by a whole-body scan using dual-energy X-ray absorptiometry. The hOGG1 overexpression in mitochondria increased the abundance of intracellular free radicals and major deletions in mtDNA. Obesity in hOGG1 TG mice resulted from increased fat content in tissues, produced by hyperphagia. The molecular mechanisms of obesity involved overexpression of genes in the central orexigenic (appetite-stimulating) pathway, peripheral lipogenesis, down-regulation of genes in the central anorexigenic (appetite-suppressing) pathway, peripheral adaptive thermogenesis, and fatty acid oxidation. Diffuse hepatosteatosis, female infertility, and increased frequency of malignant lymphoma were also seen in these hOGG1 TG mice. High levels of hOGG1 expression in mitochondria, resulting in enhanced oxidative DNA damage processing, may be an important factor in human metabolic syndrome, infertility, and malignancy.

High-performance liquid chromatography electrospray ionization tandem mass spectrometry for the detection and quantitation of pyrrolizidine alkaloid-derived DNA adducts in vitro and in vivo.

Pyrrolizidine alkaloid-containing plants are widespread in the world and are probably the most common poisonous plants affecting livestock, wildlife, and humans. Pyrrolizidine alkaloids require metabolism to exert their genotoxicity and tumorigenicity. We have determined that the metabolism of a series of tumorigenic pyrrolizidine alkaloids in vitro or in vivo generates a common set of (+/-)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-derived DNA adducts that are responsible for tumor induction. The identification and quantitation of the DHP-derived DNA adducts formed in vivo and in vitro were accomplished previously by (32)P-postlabeling/HPLC methodology. In this article, we report the development of a sensitive and specific liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC-ES-MS/MS) method to detect DHP-derived DNA adducts formed in vitro and in vivo. The method is used to quantify the levels of DHP-2'-deoxyguanosine (dG) and DHP-2'-deoxyadenosine (dA) adducts by multiple reaction monitoring (MRM) analysis in the presence of known quantities of isotopically labeled DHP-dG and DHP-dA internal standards. This HPLC-ES-MS/MS method is accurate and precise. When applied to liver samples from rats treated with the pyrrolizidine alkaloids riddelliine and monocrotaline, the method provided significant new information regarding the mechanism of DNA adduct formation.

Formation of DHP-derived DNA adducts from metabolic activation of the prototype heliotridine-type pyrrolizidine alkaloid, heliotrine.

Pyrrolizidine alkaloid-containing plants are widespread in the world and may be the most common poisonous plants affecting livestock, wildlife, and humans. Pyrrolizidine alkaloids require metabolism to exert their genotoxicity and tumorigenicity. Our mechanistic studies have determined that metabolism of the retronecine-type (riddelliine, retrorsine, and monocrotaline), heliotridine-type (lasiocarpine), and otonecine-type (clivorine) tumorigenic pyrrolizidine alkaloids in vivo and/or in vitro all generates a common set of 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-derived DNA adducts responsible for tumor induction. All the pyrrolizidine alkaloids studied previously are diesters with an ester linkage at the C7 and C9 positions of the necine base. In this study, we report that F344 rat liver microsomal metabolism of heliotrine, a tumorigenic monoester bearing a hydroxyl group at the C7 of the necine base, resulted in the formation of the dehydroheliotridine (DHH) metabolite. When incubations of heliotrine were carried out in the presence of calf thymus DNA, the same set of DHP-derived DNA adducts was formed. These results support that DHP-derived DNA adducts are potential common biomarkers of pyrrolizidine alkaloid exposure and tumorigenicity. For comparison, the dehydroretronecine (DHR)-derived DNA adducts formed from metabolism of riddleiine, retrorsine, monocrotaline, riddelleiine N-oxide, and retrorsine N-oxide were measured in parallel; the levels of DHP-derived DNA adduct formation were in the order: riddelliine approximately retrorsine>monocrotaline>retrorsine N-oxide>or=riddelliine N-oxide>heliotrine.

Formation of DHP-derived DNA adducts from metabolic activation of the prototype heliotridine-type pyrrolizidine alkaloid, lasiocarpine.

Pyrrolizidine alkaloids (PAs) are probably the most common poisonous plants affecting livestock, wildlife, and humans. The PAs that have been found to be tumorigenic in experimental animals belong to the retronecine-, heliotridine-, and otonecine-type PAs. Our recent mechanistic studies indicated that riddelliine, a tumorigenic retronecine-type PA, induced tumors via a genotoxic mechanism mediated by the formation of 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-derived DNA adducts. The same adducts were formed from clivorine, a tumorigenic otonecine-type PA from metabolism of clivorine by rat liver microsomes in the presence of calf thymus DNA. In this study, we report that metabolism of lasiocarpine, the prototype heliotridine PA, by F344 rat liver microsomes resulted in the formation of DHP. When incubated in the presence of calf thymus DNA, the same DHP-derived DNA adducts were formed. These results suggest that these DHP-derived DNA adducts are potential biomarkers of exposure and tumorigenicity for all types of PAs.

Metabolic activation of the tumorigenic pyrrolizidine alkaloid, monocrotaline, leading to DNA adduct formation in vivo.

Monocrotaline is a representative naturally occurring genotoxic pyrrolizidine alkaloid. Metabolism of monocrotaline by liver microsomes of F344 female rats generated (+/-)6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP) and monocrotaline-N-oxide as major metabolites. Metabolism in the presence of triacetyleandomycin, a P450 3A enzyme inhibitor, reduced the formation of DHP by 52% and monocrotaline N-oxide formation by 59%. Dexamethasone significantly induced microsomal monocrotaline metabolizing enzyme activities in rat liver and lung. Previously, we have identified a set of DHP-derived DNA adducts from DHP-modified calf thymus DNA by (32)P-post labeling/HPLC analysis. Metabolism of monocrotaline in the presence of calf thymus DNA resulted in a similar set of DHP-DNA adducts. These DHP-DNA adducts were also found in the liver DNA of rats treated with monocrotaline. The time course of the DHP-derived DNA adduct formation and removal in the liver of rats gavaged with a single dose (10mg/kg) of monocrotaline was similar to that of rats treated with riddelliine. The levels of DHP-DNA adducts in liver DNA of rats treated with monocrotaline were much lower than that of riddelliine-treated rats. Results from this study indicate that (i) DHP is a common reactive metabolite for retronecine-type of pyrrolizidine alkaloids, (ii) the formation of DHP-derived DNA adducts in the liver DNA of rats treated with monocrotaline suggests that monocrotaline-induced tumorigenicity is through a genotoxic mechanism.

Human liver microsomal reduction of pyrrolizidine alkaloid N-oxides to form the corresponding carcinogenic parent alkaloid.

Retronecine-based pyrrolizidine alkaloids, such as riddelliine, retrorsine, and monocrotaline, are toxic to domestic livestock and carcinogenic to laboratory rodents. Previous in vitro metabolism studies showed that (+/-)6,7-dihydro-7-hydroxy-1-(hydroxymethyl)-5H-pyrrolizine (DHP) and pyrrolizidine alkaloid N-oxides were the major metabolites of these compounds. DHP is the reactive metabolite of pyrrolizidine alkaloids and pyrrolizidine alkaloid N-oxides are generally regarded as detoxification products. However, a previous study of rat liver microsomal metabolism of riddelliine N-oxide demonstrated that DHP and its parent compound, riddelliine, were generated as the major metabolites of riddelliine N-oxide. In this study the metabolic activation of the three retronecine-based pyrrolizidine alkaloid N-oxides by human liver microsomes is investigated under oxidative and hypoxic conditions. Results shows that both the DHP and the corresponding parent pyrrolizidine alkaloids are the major metabolites of the human liver microsomal metabolism of pyrrolizidine alkaloid N-oxides. Under oxidative conditions, reduction of the N-oxide to pyrrolizidine alkaloid is inhibited and while under hypoxic conditions, DHP formation is dramatically decreased. The oxidative and reductive products generated from the metabolism of pyrrolizidine alkaloid N-oxides are substrate-, enzyme- and time-dependent. In the presence of troleandomycin, a microsomal CYP3A inhibitor, DHP formation is inhibited by more than 70%, while the N-oxide reduction was not affected. The level of microsomal enzyme activity in human liver is comparable with rats. The rate of in vitro metabolism by either human and rat liver microsomes follows the order of riddelliine > or = retrorsine > monocrotaline, and DHP-derived DNA adducts are detected and quantified by 32P-postlabeling/HPLC analysis. Similar DHP-derived DNA adducts are found in liver DNA of F344 rats gavaged with the pyrrolizidine alkaloid N-oxides (1.0 mg/kg). The levels of in vivo DHP-DNA adduct formation is correlated with the level of in vitro DHP formation. Our results indicate that pyrrolizidine alkaloid N-oxides may be hepatocarcinogenic to rats through a genotoxic mechanism via the conversion of the N-oxides to their corresponding parent pyrrolizidine alkaloids, and these results may be relevant to humans.

Metabolic activation of the tumorigenic pyrrolizidine alkaloid, retrorsine, leading to DNA adduct formation in vivo.

Pyrrolizidine alkaloids are naturally occurring genotoxic chemicals produced by a large number of plants. The high toxicity of many pyrrolizidine alkaloids has caused considerable loss of free-ranging livestock due to liver and pulmonary lesions. Chronic exposure of toxic pyrrolizidine alkaloids to laboratory animals induces cancer. This investigation studies the metabolic activation of retrorsine, a representative naturally occurring tumorigenic pyrrolizidine alkaloid, and shows that a genotoxic mechanism is correlated to the tumorigenicity of retrorsine. Metabolism of retrorsine by liver microsomes of F344 female rats produced two metabolites, 6, 7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP), at a rate of 4.8 +/- 0.1 nmol/mg/min, and retrorsine-N-oxide, at a rate of 17.6 +/- 0.5 nmol/mg/min. Metabolism was enhanced 1.7-fold by using liver microsomes prepared from dexamethasone-treated rats. DHP formation was inhibited 77% and retrorsine N-oxide formation was inhibited 29% by troleandomycin, a P450 3A enzyme inhibitor. Metabolism of retrorsine with lung, kidney, and spleen microsomes from dexamethasone-treated rats also generated DHP and the N-oxide derivative. When rat liver microsomal metabolism of retrorsine occurred in the presence of calf thymus DNA, a set of DHP-derived DNA adducts was formed; these adducts were detected and quantified by using a previously developed 32P-postlabeling/HPLC method. These same DNA adducts were also found in liver DNA of rats gavaged with retrorsine. Since DHP-derived DNA adducts are suggested to be potential biomarkers of riddelliine-induced tumorigenicity, our results indicate that (i) similar to the metabolic activation of riddelliine, the mechanism of retrorsine-induced carcinogenicity in rats is also through a genotoxic mechanism involving DHP; and (ii) the set of DHP-derived DNA adducts found in liver DNA of rats gavaged with retrorsine or riddelliine can serve as biomarkers for the tumorigenicity induced by retronecine-type pyrrolizidine alkaloids.

Differential mutagenicity of riddelliine in liver endothelial and parenchymal cells of transgenic big blue rats.

Riddelliine is a naturally occurring pyrrolizidine alkaloid that induces liver hemangiosarcomas in rats and mice. We previously reported higher levels of DNA adducts in liver endothelial cells than in liver parenchymal cells of riddelliine-treated mice and rats [Cancer Lett. 193 (2003) 119], suggesting that the tumor specificity is due to higher levels of DNA damage in the cells that form hemangosarcomas. In the present study, we evaluated the cell-specificity of riddelliine mutagenicity in rat liver. Female transgenic Big Blue rats were treated by gavage with 0.3 mg riddelliine per kg body weight, 5 days a week for 12 weeks. One day after the last treatment, the rats were sacrificed and liver parenchymal and endothelial cell fractions were isolated and purified. DNA was extracted from the cell fractions and used to assay for mutant frequency (MF) in the cII transgene. While there was no difference in the cII MFs of liver parenchymal cells in control and riddelliine-treated rats, the cII MF of liver endothelial cells from treated rats was significantly greater than the cII MF of endothelial cells from control rats. Molecular analysis of the mutants in liver endothelial cells indicated that G:C-->T:A transversion, a mutation that is characteristically induced by riddelliine, accounted for only 9% of all mutations in control rats, but made up 17% of mutations in treated rats. In contrast, G:C-->A:T transition, the major mutation in control rats where it made up 54% of all mutations, was reduced to 40% of mutations in riddelliine-treated rats. These results suggest that the relatively high mutagenicity of riddelliine in rat liver endothelial cells may be partially responsible for the tumorigenic specificity of this agent.

Tumorigenicity of chloral hydrate, trichloroacetic acid, trichloroethanol, malondialdehyde, 4-hydroxy-2-nonenal, crotonaldehyde, and acrolein in the B6C3F(1) neonatal mouse.

The tumorigenicity of chloral hydrate (CH), trichloroacetic acid (TCA), trichloroethanol (TCE), malondialdehyde (MDA), crotonaldehyde, acrolein, and 4-hydroxy-2-nonenal (HNE) was tested in the B6C3F(1) neonatal mouse. Mice were administered i.p. injections of CH (1000, 2000, 2500, and 5000 nmol per animal), TCA (1000 and 2000 nmol), TCE (1000 and 2000 nmol), MDA (1500 and 3000 nmol), crotonaldehyde (1500 and 3000 nmol), acrolein (75 and 150 nmol), and HNE (750 and 1500 nmol) at 8 and 15 days of age. At 12 months, only male mice treated with the positive control chemicals, 4-aminobiphenyl (500 and 1000 nmol) and benzo[a]pyrene (150 and 300 nmol), had incidences of tumors in the liver significantly higher than the solvent control. Additional male mice were dosed as described above and their livers were excised at 24, 48 h, and 7 days after the final dose. Liver DNA was isolated and analyzed by 32P-postlabeling/high-performance liquid chromatography (HPLC) and HPLC/electrochemical detection for MDA-derived adduct (M(1)G) and 8-oxo-2'-deoxyguanosine (8-OHdG) formation, respectively. At 24 and 48 h after the final dose, CH- and TCA-treated mice exhibited significantly higher M(1)G levels than the controls. 8-OHdG formation was also induced by CH, TCA, and MDA. These results suggest that under these experimental conditions the B6C3F(1) neonatal mouse is not sensitive to carcinogens that induce an increase in endogenous DNA adduct formation through lipid peroxidation or oxidative stress.

Correlation of DNA adduct formation and riddelliine-induced liver tumorigenesis in F344 rats and B6C3F(1) mice.

Riddelliine is a naturally occurring pyrrolizidine alkaloid that induces liver hemangiosarcomas in male and female F344 rats and male B6C3F(1) mice. We previously reported that eight dehydroretronecine (DHR)-derived DNA adducts were formed in liver DNA of rats treated with riddelliine. In order to examine the relationship between DNA adduct levels and the incidence of hemangiosarcomas, we have measured DHR-derived DNA adduct levels in purified rat and mouse liver endothelial cells, the cells of origin for the hemangiosarcomas. F344 rats and B6C3F(1) mice were treated by gavage 5 days per week for 2 weeks with riddelliine at 1.0 mg/kg for rats and 3.0 mg/kg for mice. One, 3, 7, and 28 days after the last dose, liver parenchymal and endothelial cell fractions were isolated, and the quantities of DHR-derived DNA adducts were determined by (32)Ppostlabeling/HPLC. The DHR-derived DNA adduct levels in the endothelial cells were significantly greater than in the parenchymal cells. The DNA adduct levels in rat endothelial cells were greater than in the mouse endothelial cells. These results indicate that the levels of riddelliine-induced DNA adducts in specific populations of liver cells correlate with the preferential induction of liver hemangiosarcomas by riddelliine.

Correlation of DNA adduct formation and riddelliine-induced liver tumorigenesis in F344 rats and B6C3F1 mice [Cancer Lett. 193 (2003) 119-125.

Riddelliine is a naturally occurring pyrrolizidine alkaloid that induces liver hemangiosarcomas in male and female F344 rats and male B6C3F1 mice. We previously reported that eight dehydroretronecine (DHR)-derived DNA adducts were formed in liver DNA of rats treated with riddelliine. In order to examine the relationship between DNA adduct levels and the incidence of hemangiosarcomas, we have measured DHR-derived DNA adduct levels in purified rat and mouse liver endothelial cells, the cells of origin for the hemangiosarcomas. F344 rats and B6C3F1 mice were treated by gavage 5 days per week for 2 weeks with riddelliine at 1.0 mg/kg for rats and 3.0 mg/kg for mice. One, 3, 7, and 28 days after the last dose, liver parenchymal and endothelial cell fractions were isolated, and the quantities of DHR-derived DNA adducts were determined by 32P-postlabeling/HPLC. The DHR-derived DNA adduct levels in the endothelial cells were significantly greater than in the parenchymal cells. The DNA adduct levels in rat endothelial cells were greater than in the mouse endothelial cells. These results indicate that the levels of riddelliine-induced DNA adducts in specific populations of liver cells correlate with the preferential induction of liver hemangiosarcomas by riddelliine.

Immunotoxicity of aflatoxin B1 in rats: effects on lymphocytes and the inflammatory response in a chronic intermittent dosing study.

We investigated the effects of aflatoxin B1 (AFB1) on isolated splenic lymphocytes and the histo-morphologic changes in the spleens and liver of Fisher-344 male rats. Weaned animals were fed chow diets that contained 0, 0.01, 0.04, 0.4, or 1.6 ppm AFB1, using an intermittent dosing regimen (4 weeks on and 4 weeks off AFB1), for 40 weeks. An additional group of animals was fed the 1.6 ppm AFB1 diet continuously. The intermittent dosing regimen was designed to evaluate effects of cumulative dose and exposure for risk assessment comparisons. The percentages of T and B cells were affected as shown by flow cytometric analysis after the dosing cycles. The observed changes appeared to reverse or compensate to some extent after the off cycles. Lymphocytes were stimulated in culture for analysis of the production of IL-2, IL-1, and IL-6. Significantly increased production of IL-1 and IL-6 was seen in the second dosing cycle (12 weeks) and the second "off" cycle (16 weeks) at the higher doses. Inflammatory infiltrates were seen in the liver after eight weeks of continuous and intermittent dosing and were increased in size and number at 12 weeks in both 1.6 ppm dose groups correlating with the peak production of Il-1 and IL-6. We concluded that AFB1 effects on the immune system can be either stimulatory or suppressive dependent on a critical exposure window of dose and time. Immune cells in spleen such as T-lymphocytes and macrophages, both important mediators of inflammatory responses to tissue damage, were affected differently in the continuous and intermittent exposures to AFB1.

Riddelliine N-oxide is a phytochemical and mammalian metabolite with genotoxic activity that is comparable to the parent pyrrolizidine alkaloid riddelliine.

Pyrrolizidine alkaloids (PAs) and their N-oxide derivatives are naturally-formed genotoxic phytochemicals that are widely distributed throughout the world. Although, the quantities of PAs and PA N-oxides in plants are nearly equal, the biological and genotoxic activities of PA N-oxides have not been studied extensively. PA N-oxides are major metabolites of PAs and are generally regarded as detoxification products. However, in this study, we report that rat liver microsomes converted riddelliine N-oxide to the genotoxic 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP) metabolite. Metabolism of riddelliine N-oxide by rat liver microsomes under hypoxic conditions (argon) generated predominantly the parent PA, riddelliine. The reduction of riddelliine N-oxide to riddelliine was diminished, when the metabolism of riddelliine N-oxide with rat liver microsomes was conducted aerobically. Rat liver microsomal incubations of riddelliine N-oxide in the presence of calf thymus DNA produced a set of DHP-derived DNA adducts as detected and quantified by 32P-postlabeling/HPLC. The same DHP-derived DNA adducts were also found in liver DNA of F344 rats fed riddelliine N-oxide or riddelliine. When rats received doses of 1.0 mg/kg riddelliine N-oxide for three consecutive days, the level of DNA adducts was 39.9 +/- 0.6 adducts/10(7) nucleotides, which was 2.6-fold less than that measured in rats treated with riddelliine at the same dose. We have previously shown that these DHP-derived DNA adducts are produced by chronic feeding of riddelliine and that the adduct levels correlated with liver tumor formation. Results presented in this paper indicate that riddelliine N-oxide, through its conversion to riddelliine, is also a potential genotoxic hepatocarcinogen.

Formation of DHP-derived DNA adducts in vivo from dietary supplements and chinese herbal plant extracts containing carcinogenic pyrrolizidine alkaloids.

We recently determined that the metabolism of a series of tumorigenic pyrrolizidine alkaloids resulted in the formation of a set of 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-derived DNA adducts. These DHP-derived DNA adducts have been proposed as potential biomarkers of pyrrolizidine alkaloid tumorigenicity, as well as pyrrolizidine alkaloid exposure. In this paper, we report that DHP-derived DNA adducts are formed in the liver of female F344 rats, gavaged with three dietary supplements (comfrey root extract, comfrey compound oil, and coltsfoot root extract), or an extract of a Chinese herbal plant, flos farfara (Kuan Tong Hua).

Photoirradiation of representative polycyclic aromatic hydrocarbons and twelve isomeric methylbenz[a]anthracene with UVA light: formation of lipid peroxidation.

Polycyclic aromatic hydrocarbons (PAHs) are widespread genotoxic environmental pollutants, which require metabolic activation in order to exert biological activities, including mutagenicity and carcinogenicity. Photoactivation is another activation pathway that can lead to PAH genotoxicity. In this paper, we demonstrate that photoirradiation of a series of representative PAHs, with and without bearing a methyl substituent, with UVA light in the presence of methyl linoleate resulted in the formation of methyl linoleate hydroperoxides (a lipid peroxide). The lipid peroxide formation was inhibited by dithiothreitol (DTT) (free radical scavenger), NaN3 (singlet oxygen and free radical scavenger), and superoxide dismutase (SOD) (superoxide scavenger), but was enhanced by the presence of deuterium oxide (D2O) (extends singlet oxygen lifetime). These results suggest that photoirradiation of PAHs by UVA light generates reactive oxygen species (ROS), which induce lipid peroxidation.

Metabolic formation of DHP-derived DNA adducts from a representative otonecine type pyrrolizidine alkaloid clivorine and the extract of Ligularia hodgsonnii hook.

Plants that contain pyrrolizidine alkaloids (PAs) are widely distributed, and PAs have been shown to be genotoxic and tumorigenic in experimental animals. Our recent mechanistic studies indicated that riddelliine, a tumorigenic retronecine type PA, induced tumors via a genotoxic mechanism mediated by the formation of a set of eight 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-derived DNA adducts. However, it is not known if this mechanism is general to PAs of other types. In this study, we report that the metabolism of clivorine, a tumorigenic otonecine type PA, by F344 rat liver microsomes results in DHP formation. When incubations were conducted with clivorine in the presence of calf thymus DNA, eight DHP-derived DNA adducts were formed. The Ligularia hodgsonnii Hook plant, an antitussive traditional Chinese medicine, was found to contain otonecine type PAs with clivorine being predominant. DHP and DHP-derived DNA adducts were also obtained when microsomal incubations were conducted with extracts of L. hodgsonnii Hook. This is the first report that DHP-derived DNA adducts are formed from the metabolic activation of otonecine type PA and that these DHP-derived DNA adducts are potential biomarkers of PA exposure and PA-induced tumorigenicity. These results also provide evidence that the principal metabolic activation pathway of clivorine leading to liver genotoxicity and tumorigenicity is (i) formation of the corresponding dehydropyrrolizidine (pyrrolic) derivative through oxidative N-demethylation of the necine base followed by ring closure and dehydration and (ii) binding of the pyrrolic metabolite to DNA leading to the DNA adduct formation and tumor initiation.

Mutations induced by the carcinogenic pyrrolizidine alkaloid riddelliine in the liver cII gene of transgenic big blue rats.

Riddelliine is a naturally occurring pyrrolizidine alkaloid that forms a number of different mononucleotide and dinucleotide adducts in DNA. It is a rodent carcinogen and a potential human hazard via food contamination. To examine the mutagenicity of riddelliine, groups of six female transgenic Big Blue rats were gavaged with 0.1, 0.3, and 1.0 mg riddelliine per kg body weight. The middle and high doses resulted in liver tumors in a previous carcinogenesis bioassay. The animals were treated 5 days a week for 12 weeks and sacrificed 1 day after the last treatment. The liver DNA was isolated for analysis of the mutant frequency (MF) in the transgenic cII gene, and the types of mutations were characterized by sequencing the mutants. A significant dose-dependent increase in MF was found, increasing from 30 x 10(-)(6) in the control animals to 47, 55, and 103 x 10(-)(6) in the low, middle, and high dose groups, respectively. Molecular analysis of the mutants indicated that there was a statistically significant difference between the mutational spectra from the riddelliine-treated and the control rats. A G:C --> T:A transversion (35%) was the major type of mutation in rats treated with riddelliine, whereas a G:C --> A:T transition (55%) was the predominant mutation in the controls. In addition, mutations from the riddelliine-treated rats included an unusually high frequency (8%) of tandem base substitutions of GG --> TT and GG --> AT. These results indicate that riddelliine is a genotoxic carcinogen in rat liver and that the types of mutations induced by riddelliine are consistent with riddelliine adducts involving G:C base pairs.

Human liver microsomal metabolism and DNA adduct formation of the tumorigenic pyrrolizidine alkaloid, riddelliine.

Riddelliine, a widespread naturally occurring genotoxic pyrrolizidine alkaloid, induced liver tumors in rats and mice in an NTP 2-year carcinogenicity bioassay. We have determined that riddelliine induces liver tumors in rats through a genotoxic mechanism involving the formation of (+/-)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP), which reacts with DNA to form a set of eight DNA adducts. To determine the relevance to humans of the results obtained in experimental animals, the metabolism of riddelliine was conducted using human liver microsomes. As with rat liver microsomes, DHP and riddelliine N-oxide were major metabolites in incubations conducted with human liver microsomes. The levels of DHP and riddelliine N-oxide were 0.20-0.62 and 0.03-0.15 nmol/min/mg protein, respectively, which are comparable to those obtained from rat liver microsomal metabolism. When metabolism was conducted in the presence of calf thymus DNA, the same set of eight DHP-derived DNA adducts was formed. Both the metabolism pattern and DNA adduct profile were very similar to those obtained from rat liver microsomes. When metabolism was conducted in the presence of the P450 3A4 enzyme inhibitor triacetyleandomycin, the formation of DHP and riddelliine N-oxide was reduced 84 and 92%, respectively. For DHP formation, the Km values were determined to be 0.37 +/- 0.05 and 0.66 +/- 0.08 mM from female rats and female humans; the Vmax values from female rat and human liver microsomal metabolism were 0.48 +/- 0.03 and 1.70 +/- 0.09 nmol/min/mg protein, respectively. These results strongly indicate the mechanistic data on liver tumor induction obtained for riddelliine in laboratory rodents is highly relevant to humans.

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.

Toxicokinetics of riddelliine, a carcinogenic pyrrolizidine alkaloid, and metabolites in rats and mice.

Riddelliine is a representative pyrrolizidine alkaloid, a class of naturally occurring toxic phytochemicals present in plant species worldwide. Human exposure to pyrrolizidine alkaloids occurs through consumption of herbal dietary supplements, including comfrey, and through contaminated livestock products (e.g., milk). A recently completed 2-year bioassay of riddelliine carcinogenicity showed that male and female rats and male mice, but not female mice, developed liver tumors. The toxicokinetics of riddelliine and two metabolites, the N-oxide and retronecine, were determined in serum following an oral gavage dose in male and female rats and mice using a validated liquid chromatography-electrospray mass spectrometric method. The results are consistent with extensive metabolism of riddelliine and its more polar metabolites prior to excretion. It is concluded that factors other than toxicokinetics are responsible for the observed species/sex specificity of gross toxicity or liver tumor induction in rats and mice.