Protective effects of butylated hydroxyanisole against the acute toxicity of monocrotaline in mice

1-Formyl-7-hydroxy-6,7-dihydro-5 H-pyrrolizine (1-CHO-DHP): A Potential Proximate Carcinogenic Metabolite of Pyrrolizidine Alkaloids

Pyrrolizidine alkaloids (PAs) are phytochemicals present in more than 6000 plant species worldwide; about half of the PAs are hepatotoxic, genotoxic, and carcinogenic. Because of their wide exposure and carcinogenicity, the International Programme on Chemical Safety (IPCS) concluded that PAs are a threat to human health and safety. We recently determined that PA-induced liver tumor initiation is mediated by a set of four (±)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5 H-pyrrolizine (DHP)-DNA adducts and proposed that these DHP-DNA adducts are biomarkers of PA exposure and liver tumor initiation. To validate the generality of this metabolic activation pathway and DHP-DNA adducts as biomarkers, it is significant to identify reactive metabolites associated with this metabolic activation pathway. Segall et al. ( Segall et al. ( 1984 ) Drug Metab. Dispos. 12 , 68 - 71 ) previously reported that 1-formyl-7-hydroxy-6,7-dihydro-5 H-pyrrolizine (1-CHO-DHP) is generated from the metabolism of senecionine by mouse liver microsomes. In the present study, we examined the metabolism of seven hepatocarcinogenic PAs (senecionine, intermedine, retrorsine, riddelliine, DHR, heliotrine, and senkirkine) and one noncarcinogenic PA (platyphylline) by human, rat, and mouse liver microsomes. 1-CHO-DHP was identified as a common metabolite from the metabolism of these hepatotoxic PAs, but not from platyphylline. Incubation of 1-CHO-DHP with HepG2 and A549 cells produced the same set of DHP-DNA adducts, which were identified by both LC/MS MRM mode and selected ion monitoring analyses through comparison to synthetic standards. In the incubation medium of 1-CHO-DHP treated HepG2 cells, both DHP and 7-cysteine-DHP were formed, which were capable of binding to cellular DNA to produce DHP-DNA adducts. These results suggest that 1-CHO-DHP is a proximate DNA metabolite of genotoxic and carcinogenic PAs.

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.

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.

The effects of exogenous glutathione and cysteine on oxidative stress induced by ferric nitrilotriacetate

The effects of the antioxidants, glutathione (GSH) and its precursor cysteine (Cys) on oxidative damage induced by ferric nitrilotriacetate (Fe-NTA) were examined. Fe-NTA-associated oxidative stress caused the depletion of renal cellular GSH content. Administration of exogenous GSH and Cys suppressed 8-hydroxydeoxyguanosine (8-OH-dG) formation, an indicator of oxidative DNA damage and nephrotoxicity following Fe-NTA treatment. This suggests that generation of free radicals may be causally involved in oxidative lesion generation. Since lipid peroxidation was found to be inhibited only by GSH and not Cys treatment, this suggests that this effect and the DNA damage might be mediated by different pathways. Fe-NTA-associated oxidative stress in renal tubular cells might thus operate via both intracellular and external space modes.

Identity of a biliary metabolite formed from monocrotaline in isolated, perfused rat liver

A pneumotoxic pyrrolic metabolite, previously isolated from the bile when rat liver was perfused with the pyrrolizidine alkaloid, monocrotaline, has been identified as 7-glutathionyl-dehydroretronecine. The metabolite showed a TLC spot and HPLC peak corresponding with the latter compound, and a procedure for replacing the thioether group with an ethoxy group converted the metabolite to dehydroretronecine 7-ethyl ether, confirming that the glutathionyl moiety was attached to the 7-position of dehydroretronecine. The same metabolite was detected in bile from rat liver perfused with retrorsine, which is a diester alkaloid similar to monocrotaline, whereas it was not formed from heliotrine, an alkaloid lacking the 7-ester function.

Effects of dietary pyrrolizidine (Senecio) alkaloids on vitamin A metabolism in rats

In two experiments, the effect of feeding the pyrrolizidine alkaloid (PA)-containing plant tansy ragwort (Senecio jacobaea) on the metabolism of vitamin A in rats was examined. In Experiment 1, dietary levels of 0, 5 and 10% tansy ragwort and 0, 25,000 and 100,000 IU supplementary vitamin A/kg diet were used. In rats fed tansy ragwort, both plasma and liver concentrations of vitamin A were depressed (P less than 0.05). Plasma values were decreased to about 50% of control values. In Experiment 2, rats fed 5% tansy ragwort had depressed plasma and liver vitamin A concentrations 48 h after oral dosing with vitamin A. Fecal excretion of vitamin A was decreased in tansy ragwort-fed rats. In control rats, most fecal vitamin A was excreted in the first 24 h post-dosing, while in tansy ragwort-fed animals, the excretion was delayed, suggesting a possible effect of PA on gut motility. The results indicate that PA causes reductions in liver and plasma vitamin A concentrations. Postulated mechanisms include an effect of PA on hepatic retinol-binding protein synthesis, and impaired biliary excretion depressing vitamin A absorption.

Monocrotaline pneumotoxicity in mice

Lung injury induced in rats by the pyrrolizidine alkaloid monocrotaline is a well-documented model of pulmonary hypertension. To our knowledge, however, monocrotaline-induced cardiopulmonary injury has rarely been described and has never been quantitated in mice. In the present study, adult male mice received 2.4, 4.8, or 24.0 mg monocrotaline/kg body weight/day in the drinking water continuously for 6 weeks. These doses represent 1, 2, and 10 times the severely pneumotoxic regimen in rats. Pulmonary endothelial function was monitored by right lung angiotensin converting enzyme (ACE) activity, plasminogen activator (PLA) activity, and prostacyclin (PGI2) and thromboxane (TXA2) production. Light and electron microscopy were performed on the left lungs. Cardiac right ventricular hypertrophy was evaluated by the right ventricle to left ventricle plus septum weight ratio (RV/LV + S). Monocrotaline-treated mice exhibited a dose-dependent decrease in lung ACE and PLA activities and an increase in PGI2 and TXA2 production, indicative of endothelial dysfunction. However, these responses were significant only after the highest monocrotaline dose. Light and electron microscopy revealed dose-dependent pulmonary inflammatory and exudative reactions. Unlike previous studies in rats, however, monocrotaline-treated mice developed relatively little lung fibrosis, cardiomegaly, or right ventricular hypertrophy, and no occlusive medial thickening of the pulmonary arteries, even at the highest dose level. These and previous data indicate that there are quantitative biochemical and qualitative morphological differences between mice and rats with respect to monocrotaline pneumotoxicity. Furthermore, in monocrotaline-treated mice (but not in rats) there appears to be a dissociation between lung endothelial dysfunction and inflammation on the one hand, and pulmonary hypertension and fibrosis on the other.

Effects of dietary butylated hydroxyanisole and cysteine on toxicity of Lathyrus odoratus to broiler and Japanese quail chicks

The effect of .75% dietary butylated hydroxyanisole (BHA) and 1% cysteine on the toxicity of Lathyrus odoratus seed to Japanese quail and broiler chicks was examined. In both species, the feeding of Lathyrus seed as a component of a complete diet depressed (P less than .05) body weight gain and feed intake. Typical signs of lathyrism, including ruffled feathers, enlarged hocks, curled toes, ataxia, leg paralysis, and mortality, were observed. Neither BHA nor cysteine exerted protective effects against the lathyrogenic effects. As assessed by mortality, these additives appeared instead to potentiate Lathyrus toxicity.

Method for determination of pyrrolizidine alkaloids and their metabolites by high-performance liquid chromatography

An improved method utilizing reverse-phase liquid chromatography on a styrene-divinylbenzene column (PRP-1) and ultraviolet detection was developed for the simultaneous determination of the pyrrolizidine alkaloids (PAs) senecionine, seneciphylline, and retrorsine and their major metabolites produced during in vitro transformation of PAs by microsomal enzyme systems. The procedure employs direct injection of the 46,000g supernatant of the microsomal reaction mixture directly onto the column, and elution with a 0.1 M NH4OH-acetonitrile gradient. The method is very gentle, simple, and fast with excellent precision since no prior extraction, preconcentration, or derivatization steps are required. Using this procedure 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine and the corresponding PA N-oxides were shown to be the major microsomal metabolites of the PAs examined. The detection limit of these metabolites was approximately 1 microM.

trans-4-Hydroxy-2-hexenal: a reactive metabolite from the macrocyclic pyrrolizidine alkaloid senecionine

The toxicity of macrocyclic pyrrolizidine alkaloids in the livers of man and animals has been attributed to the formation of reactive pyrroles from dihydropyrrolizines. Now a novel metabolite, trans-4-hydroxy-2-hexenal, has been isolated from the macrocyclic pyrrolizidine alkaloid senecionine, in an in vitro hepatic microsomal system. Other alkenals such as trans-4-hydroxy-2-nonenal have previously been isolated from microsomal systems when treated with halogenated hydrocarbons or subjected to lipid peroxidation. The in vivo pathology caused by trans-4-hydroxy-2-hexenal appears to be identical to that previously attributed to reactive pyrroles. There are similarities between the toxic effects of this alkenal and those of centrilobular hepatotoxins such as CCl4 and other alkenals formed during lipid peroxidation.

Carcinogenicity and modification of the carcinogenic response by BHA, BHT, and other antioxidants

Carcinogenicity tests showed that addition of the antioxidant BHA to the diet of F344 rats induced high incidences of papilloma and squamous cell carcinoma of the forestomach of both sexes. Male hamsters given BHA for 24 weeks also developed papilloma showing downward growth into the submucosa of the forestomach. These results indicate that BHA should be classified in the category of "sufficient evidence of carcinogenicity" as judged by IARC criteria. The 3-tert isomer of BHA seemed to be responsible for the carcinogenicity of crude BHA in the forestomach of rats. BHT was not found to be carcinogenic in rats or mice. In two-stage carcinogenesis in rats after appropriate initiation, BHA enhanced carcinogenesis in the forestomach and urinary bladder of rats, but inhibited carcinogenesis in the liver. BHT enhanced the induction of urinary bladder tumors and inhibited that of liver tumors, but had no effect on carcinogenesis in the forestomach. BHT could be a promoter of thyroid carcinogenesis. Sodium L-ascorbate enhanced forestomach and urinary bladder carcinogenesis. Ethoxyquin enhanced kidney and urinary bladder carcinogenesis, but inhibited liver carcinogenesis. Thus, these antioxidants modify two-stage chemical carcinogenesis in the forestomach, liver, kidney, urinary bladder, and thyroid, but show organ-specific differences in effects.

Protective role of dietary butylated hydroxyanisole against chemical-induced acute liver damage in mice

Prior consumption of a diet containing the food antioxidant, butylated hydroxyanisole (BHA), by female mice prevented the development of or minimized the acute liver damage caused by monocrotaline, acetaminophen, or bromobenzene. In contrast, neither the incidence nor the severity of carbon tetrachloride-induced hepatotoxicity was affected by dietary BHA. Hepatotoxicity was judged by plasma alanine aminotransferase and aspartate aminotransferase levels, hepatic cytochrome P-450 content, and liver histology. The protective effect of BHA against acetaminophen-induced hepatotoxicity was not demonstrated in male mice. The observed protection by dietary BHA against acetaminophen- and bromobenzene-induced hepatotoxicity was associated with the increase of liver glutathione. It is concluded that the protective action of BHA is dependent upon the nature of the toxic agent.

Elimination of adverse effects of ethoxyquin (EQ) by methionine hydroxy analog (MHA). Protective effects of EQ and MHA for bitterweed poisoning in sheep

Dietary ethoxyquin (EQ) and methionine hydroxy analog (MHA) protected 6-8-month-old wethers from toxic doses of bitterweed (Hymenoxys odorata DC.). The EQ-MHA group received sweet feed (corn, oats, dehydrated alfalfa pellets, cane molasses and minerals), 500 g/day/sheep, supplemented with EQ and MHA (0.5% and 1.0% of feed, respectively) for 9 days prior to the poisoning with bitterweed while the MHA group received the same feed without EQ and controls received the same amount of feed with no additives. Two of 6 MHA-treated and 3 of 7 controls died whereas all 7 EQ-MHA-treated sheep survived after receiving 5 doses of bitterweed (5 X 5.5 g/kg) in 6 days. Coadministration of MHA and EQ eliminated the adverse effect of EQ; dietary EQ lowered the serum albumin, calcium, and alkaline phosphatase content while protecting the animals from bitterweed poisoning. EQ is the most promising protective agent tested for bitterweed poisoning in sheep.

Effect of dietary butylated hydroxyanisole on methylazoxymethanol acetate-induced toxicity in mice

Administration of butylated hydroxyanisole (BHA), a widely used food additive, has been found to inhibit the carcinogenic and toxic effects of various chemicals in animal models. To study the relationship of dietary BHA to the acute toxicity of methylazoxymethanol (MAM) acetate, a colon-specific carcinogenic compound, groups of female CF1 mice were fed NIN-07 diet containing 0, 300, 1000, 3000 or 6000 ppm BHA or a semipurified diet containing 0 or 6000 ppm BHA for 4 wk, and were injected ip with MAM acetate (20 mg/kg body weight) at the end of the first 2 wk and again 4 days later. At levels of 300-6000 ppm, BHA was found to protect against death caused by MAM acetate. The mortality rates in MAM-treated mice were 80 and 92% in those fed the diets with no BHA and 0 and 1% in those fed 6000 ppm BHA, and were inversely related to the amount of BHA in the diet. The protection was associated with increased levels of hepatic cytochrome P-450 and b5 and with a reduction in necrotic changes in the liver.

Comparative effects of antioxidants on the toxicity of mixed pyrrolizidine alkaloids from Senecio jacobaea in mice

The comparative effects of the antioxidants, butylated hydroxyanisole (BHA), ethoxyquin, and cysteine on pyrrolizidine-alkaloid-induced (PA-induced) lethality and acute hepatotoxicity were assessed in female mice. Diets containing 0.75% BHA, 0.25% ethoxyquin, or 1% cysteine were fed to mice for 10 d before the ip administration of mixed PAs from Senecio jacobaea (tansy ragwort), 280 mg/kg. Without the dietary antioxidants, the PAs produced 100% mortality in 24 h. The BHA and ethoxyquin diets were completely and partially protective, respectively, against the PA-induced lethality. The deaths were associated with severe hemorrhagic lesions in liver with or without hepatocytic necrosis. Both BHA and ethoxyquin significantly reduced the incidence of the hemorrhagic lesions but not the necrotic lesions in liver. Cysteine had no significant effect on either mortality or the liver lesions induced by the mixed PAs. These results suggest that dietary antioxidants have differential protective effects against lethality and acute hepatotoxicity induced by mixed PAs from tansy ragwort.

Protective action of zinc against pyrrolizidine alkaloid-induced hepatotoxicity in rats

The influence of Zn on the acute hepatotoxicity of pyrrolizidine alkaloids (PAs) was determined in male rats. Zinc, 72 mumol/kg as ZnCl2, was administered ip for 3 consecutive days, followed 16 h after the last dose by a single ip injection of purified mixed PAs (80, 120, or 160 mg/kg) obtained from tansy ragwort (Senecio jacobaea). Hepatotoxicity of the PAs was assessed by measuring the activities of plasma glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) and by histological examination of the liver. There was a dose-dependent increase in plasma GOT and GTP 24 h after PA administration, whereas no significant increase of these enzymes was seen after administering Zn alone. The 7-fold increase in plasma GOT and 12-fold increase in GPT after PA (120 mg/kg) were reduced to 2.4- and 2.1-fold, respectively, by Zn pretreatment. The PA-induced liver necrosis was either reduced in severity or abolished by Zn when the PA dose was 80 or 120 mg/kg. These results suggest a protective effect of Zn against PA hepatotoxicity. The protective effect was associated with a marked increase in liver metallothionein and a significant decrease in hepatic cytochrome P-450 content, aminopyrine N-demethylase activity, and in vitro microsomal conversion of the PAs to pyrroles. Liver nonprotein sulfhydryls were unchanged. The possible role of metallothionein in the sequestration of pyrrole metabolites merits further investigation.

Modifications of chronic hepatotoxicity of pyrrolizidine (Senecio) alkaloids by butylated hydroxyanisole and cysteine

The chronic hepatotoxic effects of mixed pyrrolizidine alkaloids (PAs) from the poisonous plant tansy ragwort (Senecio jacobaea) and the ability of butylated hydroxyanisole (BHA) and cysteine to alter these hepatic effects were studied in male rats. In control animals, the i.p. administration of a single dose of mixed PAs, 160 mg/kg, produced marked fibrosis, biliary hyperplasia, megalocytosis, necrosis and calcification in liver 8 weeks post injection. In contrast, consumption of 0.75% BHA diet 10 days before and 14 days after PA administration reduced the incidence and/or completely prevented the occurrence of these pathological changes. Similar treatment with 1% cysteine, however, only reduced the severity of the hepatic lesions.