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

Lack of protective action of cysteine against the fetotoxic effect of monocrotaline

Monocrotaline (MCT), a pyrrolizidine alkaloid present in Crotalaria species, has hepatotoxic, nephrotoxic, pneumotoxic and fetotoxic effects. However, the toxic effects of exposure to MCT in adult rats can be prevented by cysteine. Thus, the present study was conducted to evaluate the possible prevention by cysteine of the toxic effects of MCT on pregnant rats. Thirty-six pregnant rats were used. The females in the experimental groups were fed ration containing 0.02% MCT, 0.02% MCT + 1% cysteine, or 1% cysteine from day 6 to day 21 of pregnancy; the control group was fed only common ration for the same period of time. All rats were killed on day 21 of pregnancy and their blood was collected for determination of liver and kidney function. General toxicity to pregnant dams was assessed. Fetuses were removed by caesarian section and embryofetotoxic parameters were examined. Results showed impaired body weight gain in rats fed MCT, with or without cysteine supplementation. Plasma levels of AST, ALT, LDH, GGT, urea and creatinine were increased in MCT animals compared to controls. The pathology study revealed lesions only in dams from the MCT group. The weights of the placentas and fetuses of the MCT and MCT + cysteine groups were significantly lower than those of the control group. Thus, the present data suggests some protective action of 1% of cysteine in ration against the toxic effects of MCT on the dams but not on the litter.

Development of a (32)P-postlabeling/HPLC method for detection of dehydroretronecine-derived DNA adducts in vivo and in vitro

Pyrrolizidine alkaloids are naturally occurring genotoxic chemicals produced by a large number of plants. Metabolism of pyrrolizidine alkaloids in vivo and in vitro generates dehydroretronecine (DHR) as a common reactive metabolite. In this study, we report the development of a (32)P-postlabeling/HPLC method for detection of (i) two DHR-3'-dGMP and four DHR-3'-dAMP adducts and (ii) a set of eight DHR-derived DNA adducts in vitro and in vivo. The approach involves (1) synthesis of DHR-3'-dGMP, DHR-3'-dAMP, and DHR-3',5'-dG-bisphosphate standards and characterization of their structures by mass and (1)H NMR spectral analyses, (2) development of optimal conditions for enzymatic DNA digestion, adduct enrichment, and (32)P-postlabeling, and (3) development of optimal HPLC conditions. Using this methodology, we have detected eight DHR-derived DNA adducts, including the two epimeric DHR-3',5'-dG-bisphosphate adducts both in vitro and in vivo.

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

Riddelliine is a representative naturally occurring genotoxic pyrrolizidine alkaloid. We have studied the mechanism by which riddelliine induces hepatocellular tumors in vivo. Metabolism of riddelliine by liver microsomes of F344 female rats generated riddelliine N-oxide and dehydroretronecine (DHR) as major metabolites. Metabolism was enhanced when liver microsomes from phenobarbital-treated rats were used. Metabolism in the presence of calf thymus DNA resulted in eight DNA adducts that were identical to those obtained from the reaction of DHR with calf thymus DNA. Two of these adducts were identified as DHR-modified 7-deoxyguanosin-N(2)-yl epimers (DHR-3'-dGMP); the other six were DHR-derived DNA adducts, but their structures were not characterized. A similar DNA adduct profile was detected in the livers of female F344 rats fed riddelliine, and a dose-response relationship was obtained for the level of the total (eight) DHR-derived DNA adducts and the level of the DHR-3'-dGMP adducts. These results suggest that riddelliine induces liver tumors in rats through a genotoxic mechanism and the eight DHR-derived DNA adducts are likely to contribute to liver tumor development.

Induction of cytochrome P450 enzymes in the livers of rats treated with the pyrrolizidine alkaloid retrorsine

Retrorsine is a member of the pyrrolizidine alkaloid (PA) family of naturally occurring compounds found in a large number of plant species worldwide. The cytotoxic, mutagenic, and antimitotic effects of PAs have made them targets for studies designed to determine their potential contributions to carcinogen esis and their usefulness for anticancer therapy. Evidence from the literature suggests that bioactivation of PAs by liver cytochrome P450 (CYP) enzymes is required for their toxicity. However, the specific CYP isozymes that are involved in retrorsine metabolism have not been identified. To address this issue, we administered retrorsine to a cohort of young adult male rats and examined induction or enhanced expression of mRNA and protein for widely studied hepatic CYP isoforms spanning four families together with the essential enzyme CYP reductase. The protein levels of normally expressed CYPs 1A2, 2B1/2, and 2E1 increase significantly in rat liver microsomes from retrorsine-treated rats compared to untreated control rats (P < 0. 05), but protein levels of CYP 4A3, CYP 3A1, and CYP reductase were unchanged after retrorsine treatment. In addition, CYP 1A1 mRNA and protein, which are not detectable in the livers of control rats, were induced after retrorsine exposure. The results of the present study demonstrate enhanced or induced expression of hepatic CYPs 1A1, 1A2, 2E1, and 2B1/2 in response to retrorsine exposure in rats, suggesting that one or more of these enzymes may be involved in retrorsine metabolism.

Pyrrolizidine alkaloids in human diet

Pyrrolizidine alkaloids are the leading plant toxins associated with disease in humans and animals. Upon ingestion, metabolic activation in liver converts the parent compounds into highly reactive electrophiles capable of reacting with cellular macromolecules forming adducts which may initiate acute or chronic toxicity. The pyrrolizidine alkaloids present a serious health risk to human populations that may be exposed to them through contamination of foodstuffs or when plants containing them are consumed as medicinal herbs. Some pyrrolizidine alkaloids (PA) adducts are persistent in animal tissue and the metabolites may be re-released and cause damage long after the initial period of ingestion. PAs are also known to act as teratogens and abortifacients. Chronic ingestion of plants containing PAs has also led to cancer in experimental animals and metabolites of several PAs have been shown to be mutagenic in the Salmonella typhimurium/mammalian microsome system. However, no clinical association has yet been found between human cancer and exposure to PAs. Based on the extensive reports on the outcome of human exposure available in the literature, we conclude that while humans face the risk of veno-occlusive disease and childhood cirrhosis PAs are not carcinogenic to humans.

Comparative DNA cross-linking by activated pyrrolizidine alkaloids

The toxicity and bioactivity of pyrrolizidine alkaloids (PAs), common constituents of hundreds of plant species, and in herbal remedies and folk medicines prepared thereof, are probably due to their ability to form DNA cross-linking. We investigated DNA cross-linking activity by chemically-activated PAs from four different structural classes in Madin-Darby bovine kidney (MDBK) cells and in pBR322 DNA. In cell culture, alpha,beta-unsaturated macrocyclic diester pyrroles dehydrosenecionine (DHSN), dehydroriddelliine (DHRD) and the saturated macrocyclic diester pyrrole dehydromonocrotaline (DHMO) were significantly more potent cross-linkers than the simple necine base (retronecine) and an N-oxide (indicine N-oxide; INO) as determined by alkaline elution. The proportion of total DNA cross-links that were proteinase K-resistant (DNA-DNA cross-links) induced by the various pyrroles ranged from 0.08 (DHRN) to 0.67 (DHSN). Those pyrroles that were potent cross-linkers of cellular DNA also cross-linked, in a dose-dependent manner, Bam HI-digested pBR322 DNA as assessed by a gel retardation assay. The possible functional relevance of pyrrole-DNA cross-links was determined by their ability to interrupt PCR amplification of a 1.129 kb segment of pBR322. Dehydrosenecionine completely inhibited amplification, while DHMO was of intermediate potency, while DHRN and INO had no effect. Taken together, these studies suggest that structural features, most notably the presence of a macrocyclic diester, confer potent cross-link activity to PAs. In any event, DNA-DNA cross-linking is probably biologically relevant as indicated by their interference with DNA replication.

Pyrrolizidine alkaloids crosslink DNA with actin

Pyrrolizidine alkaloids (PAs) are toxic constituents of hundreds of plant species, some of which people are exposed to in herbal products and traditional remedies. The bioactivity of PAs are related, at least in part, to their ability to form DNA-protein complexes (DPC). Previous studies from our laboratory indicated a possible role for actin in PA-induced DPCs. Nuclei prepared from Madin-Darby bovine kidney (MDBK) and human breast carcinoma (MCF-7) cells were treated with the pyrrolic PAs dehydrosenecionine (DHSN) and dehydromonocrotaline (DHMO). DPCs were purified and then analyzed by Western immunoblotting. Actin was found in DPCs induced by both DHSN and DHMO, but not in those from control nuclei. Actin was also present in DPCs induced by cisplatinum and mitomycin C, two bifunctional cross-linkers. In separate experiments, DHSN and DHMO were crosslinked to a mixture of HindIII digested lambda phage with varying amounts of glutathione (GSH), cysteine, or methionine to identify the stoichiometry of competition between DNA and alternate nucleophiles for crosslink formation with pyrroles. GSH and cysteine, but not methionine, competed with lambda phage for DNA crosslinking, indicating that reduced thiols may have a role in nucleophilic reactions with pyrroles in the cell. While actin involvement in cisplatinum-induced DPCs is documented, the discovery of actin crosslinking in PA or mitomycin C-treated cells or nuclei is, to our knowledge, novel. Pyrrole-induced DPC formation with actin, a protein with structural and/or regulatory importance proteins, may be a significant mechanism for PA toxicity and bioactivity.

Dehydromonocrotaline generates sequence-selective N-7 guanine alkylation and heat and alkali stable multiple fragment DNA crosslinks

Monocrotaline is a pyrrolizidine alkaloid known to cause toxicity in humans and animals. Its mechanism of biological action is still unclear although DNA crosslinking has been suggested to a play a role in its activity. In this study we found that an active metabolite of monocrotaline, dehydromonocrotaline (DHM), alkylates guanines at the N7 position of DNA with a preference for 5'-GG and 5'-GA sequences. In addition, it generates piperidine- and heat-resistant multiple DNA crosslinks, as confirmed by electrophoresis and electron microscopy. On the basis of these findings, we propose that DHM undergoes rapid polymerization to a structure which is able to crosslink several fragments of DNA.

Prolonged cell-cycle arrest associated with altered cdc2 kinase in monocrotaline pyrrole-treated pulmonary artery endothelial cells

Monocrotaline pyrrole (MCTP), a metabolite of the pyrrolizidine alkaloid monocrotaline, is thought to initiate damage to pulmonary endothelial cells resulting in delayed but progressive pulmonary interstitial edema, vascular wall remodeling, and increasing pulmonary hypertension. MCTP was previously shown to inhibit pulmonary endothelial cell proliferation and cause cell-cycle arrest in vitro. To determine the persistence of arrest and better characterize the cell-cycle stage at which it occurs, bovine pulmonary artery endothelial cells (BPAEC) under differing growth conditions were exposed to low (5 microg/ml) or high (34.5 microg/ml) concentrations of MCTP for varying times. Flow cytometric cell-cycle analysis was coupled with Western blot and biochemical analysis of cdc2 kinase and measurements of cell size. MCTP treatment induced a G2 + M phase arrest in 48-h exposed confluent BPAEC that persisted for at least 28 d and was associated with continued cellular enlargement. A short-duration MCTP exposure of confluent (low and high concentration) and log phase (high concentration) BPAEC caused persistent cell-cycle arrest for 1 wk, whereas a low-concentration exposure in log phase cells resulted in cell-cycle arrest with reversal 96 h after exposure. Western blot examination revealed that by 24 h of MCTP exposure, the phosphorylation state of cdc2 was consistent with the inactive form of the kinase (confirmed by biochemical assay); this alteration persisted through at least 96 h of exposure. We conclude that MCTP induces a progressive irreversible endothelial cell dysfunction leading to inactivation of cdc2 kinase and irreversible cell-cycle arrest at the G2 checkpoint.

Clastogenic activity of integerrimine determined in mouse micronucleus assays

The pyrrolizidine alkaloid integerrimine, obtained from Senecio brasiliensis, was tested by acute dosing at two concentrations (18.75 and 37.5 mg/kg), and at different times, to establish its ability to induce micronuclei in mouse erythrocytes. This alkaloid was able to increase the frequency of micronucleated polychromatic erythrocytes in both, bone marrow and peripheral blood erythrocytes

Hypertrophy and prolonged DNA synthesis in smooth muscle cells characterize pulmonary arterial wall thickening after monocrotaline pyrrole administration to rats

Monocrotaline pyrrole (MCTP) is a highly reactive pneumotoxic metabolite of the pyrrolizidine alkaloid plant toxin monocrotaline. When administered to rats, it causes a delayed and progressive lung injury, vascular remodeling, and pulmonary hypertension. Structural remodeling consists of endothelial cell swelling followed by increased thickness of the vascular media in small pulmonary arteries and muscularization of normally nonmuscular arteries. Experiments were performed to characterize DNA synthesis and cell proliferation in vascular smooth muscle cells (VSMCs) after MCTP and to determine their relationship to changes in the thickness of the arterial medial layer of pulmonary resistance vessels. Male Sprague-Dawley rats were treated with MCTP (3.5 mg/kg, intravenously) or its vehicle (dimethylformamide). To label cells actively synthesizing DNA, rats were given the thymidine analog, bromodeoxyuridine (BrdU), 3 times by intraperitoneal injection during the 24 hr preceding euthanasia. Using immunohistochemistry, BrdU incorporation was quantified as a ratio of labeled nuclei to total nuclei. Within 5 days after MCTP administration, the thickness of the medial smooth muscle layer in arteries 60-250 microm in diameter was increased, prior to evidence of right heart hypertrophy. BrdU incorporation by VSMCs in pulmonary arteries was not different in vehicle- and MCTP-treated rats for the first 48 hr after treatment. However, MCTP caused a significant increase in DNA synthesis in VSMC on days 3-8 in arteries up to 250 microm in diameter. Although increased DNA synthesis precedes cell proliferation, the relative number of medial VSMCs did not increase over 8 days, suggesting that hypertrophy alone was responsible for the increased thickness of the arterial media. These results demonstrate that MCTP causes thickening of the media of pulmonary vessels through VSMC hypertrophy and that the prolonged DNA synthesis that accompanies VSMC hypertrophy is not followed by proliferation.

Comparative cytotoxicity of monocrotaline and its metabolites in cultured pulmonary artery endothelial cells

Metabolites of the pyrrolizidine alkaloid monocrotaline cause progressive development of pulmonary hypertension in rats. The putative reactive intermediate monocrotaline pyrrole (MCTP) has been shown to cause cytotoxicity, hypertrophy, decreased proliferation, and altered synthetic capability in cultured pulmonary endothelial cells. We compared effects of monocrotaline (MCT) at 60 micrograms/ml (0.185 mM) with previously identified metabolites, MCTP 10 micrograms/ml (0.031 mM) and glutathione-conjugated dihydropyrrolizine (GSH-DHP) 60 micrograms/ml (0.135 mM), in cultured bovine pulmonary artery endothelial cells (BPAECs). To determine whether endothelial metabolism might contribute to the mechanism of this toxicity, we used markers of cytotoxicity (LDH release), synthetic activity (PGI2 synthesis), hypertrophy (planimetry), cell density (cell count/area), and Evans blue albumin (EBA) transudation as a marker for loss of fluid barrier integrity. We found changes in all endothelial markers with MCTP only. MCTP caused increased LDH release by 48 hr, augmented PGI2 synthesis by 96 hr, and resulted in hypertrophy and decreased cell density by 48 hr that persisted at least 21 days. There was increased EBA transudation at 24 hr posttreatment. We concluded that, based on markers of endothelial damage, BPAECs showed no apparent ability to metabolize MCT to a reactive intermediate nor to further metabolize GSH-DHP to a toxic species. We also concluded that MCTP can cause a direct effect on fluid barrier integrity of endothelial cell monolayers in the absence of inflammation.

Retrorsine in breast milk influences copper handling in suckling rat pups

AIMS

To explore the hypothesis that a second xenobiotic agent is required with excess copper to produce Indian Childhood Cirrhosis, this study investigated the effect of the pyrrolizidine alkaloid retrorsine fed to the mother during the suckling period upon the serial changes in neonatal copper status.

METHODS

Female Wistar rats with new-born litters were fed either a control or a retrorsine (50 mg/kg) diet. At 0, 4, 8, 11, 15, 18 and 21 days, pups from each litter were weighed, sacrificed and their livers removed for copper, DNA and metallothionein analysis. Serum samples were assayed for caeruloplasmin oxidase activity and albumin.

RESULTS

1) Higher than adult level of hepatic copper in normal rats which rose post-natally before declining from day 11 after birth, 2) raised hepatic copper concentrations and total copper in the retrorsine group from day 15; levels were higher than adult at birth, 3) reduced serum caeruloplasmin oxidase activity and albumin levels in retrorsine group, but both groups lower than adult, 4) lower hepatic metallothionein levels in retrorsine group, but both groups higher than adult, and 5) reduced liver DNA in the retrorsine group when expressed as total DNA and per gram of tissue. These changes were not secondary to under-nutrition as a small study on under-nourished rat neonates showed that copper handling is not significantly altered when compared to well-nourished rats.

CONCLUSIONS

Retrorsine passing to rat neonates via breast milk causes: 1) the accumulation of hepatic copper, 2) impairment of the rise in serum caeruloplasmin, which could indicate a decline in synthesis or failure of copper incorporation into the apo-protein, 3) a decrease in hepatic metallothionein and serum albumin levels, again suggesting diminished protein synthesis, and 4) reduced hepatic DNA indicative of decreased cell number but increased cell size. Accumulation of liver copper but reduction of copper-binding proteins could result in free copper and explain the synergistic hepatotoxicity of copper and retrorsine.

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.

Directly toxic effects of plant chemicals which may occur in human and animal foods

Pyrrolizidine alkaloids are among the most significant plant chemicals causing disease in animals and humans. After absorption from the gut, the compounds are converted to electrophilic pyrroles in the liver which, apart from causing damage to this organ, may escape to cause injury to extraheptic tissues such as the lungs, heart, and kidneys. A group of compounds more recently found to be associated with neurotoxicity are various polyhydroxyalkaloids which are able to interfere with polysaccharide metabolism. They are able to inhibit lysosomal monosaccharidases by virtue of their structural resemblance to the transition state of particular sugar molecules. The resulting lysosomal storage diseases have pathology identical to that of the respective congenital and heritable lysosomal storage diseases which occur in animals and humans. Consumption of cycad plants by cattle may cause a neurotoxicity characterised mainly by a posterior sensory ataxia. In recent years, cycads are considered to be a risk factor for a spectrum of progressive neuro degenerative diseases of humans in the Western Pacific region. The known toxins in the plant are the methylazoxymethanol glycosides which are hepatotoxic and carcinogenic, and the neurotoxic non-protein amino acid beta-methylaminoalanine. A plant carcinogen which can be of great abundance in the nutritional environment is the illudine norsesquiterpene glucoside ptaquiloside which is found in bracken fern. This is the only plant carcinogen which causes natural outbreaks of bladder and/or intestinal cancer in livestock. Many legumes contain phytooestrogens, notably isoflavones. Consumption of these compounds at high levels by sheep can cause extensive lesions of the genitalia of females and castrated males.(ABSTRACT TRUNCATED AT 250 WORDS)

Synergistic liver toxicity of copper and retrorsine in the rat

To investigate the possible synergy between copper and retrorsine (a pyrrolizidine alkaloid) as a cause of Indian Childhood Cirrhosis, four groups of male Wistar rats were fed the following diets from weaning: A. Normal diet; B. Copper loaded (2 g CuSO4/kg diet); C. Retrorsine supplemented (Expt 1:25 mg/kg body weight/week by gavage, Expt 2:25 mg/kg food initially then 15 mg/kg food after 4 weeks); and D. Copper and retrorsine as above. Serial plasma samples were assayed for aminotransferases, albumin and bilirubin. Liver samples at biopsy and sacrifice provided samples for copper analysis and histology. Results showed that copper and retrorsine together significantly increased liver damage compared with feeding either alone as assessed by: 1. Increased mortality rate; 2. Decreased plasma albumin and increased plasma bilirubin (mainly conjugated) indicative of hepatocyte dysfunction; 3. Massive liver copper accumulation, and 4. Increased liver damage histologically. Thus retrorsine caused liver copper accumulation, and together copper and retrorsine led to severe hepatic dysfunction, characterised by hypoalbuminaemia and conjugated hyperbilirubinaemia. Plant alkaloids secreted in milk by grazing animals and copper from brass vessels may together produce Indian Childhood Cirrhosis.

Mechanisms and pathology of monocrotaline pulmonary toxicity

Monocrotaline (MCT) is an 11-membered macrocyclic pyrrolizidine alkaloid (PA) that causes a pulmonary vascular syndrome in rats characterized by proliferative pulmonary vasculitis, pulmonary hypertension, and cor pulmonale. Current hypotheses of the pathogenesis of MCT-induced pneumotoxicity suggest that MCT is activated to a reactive metabolite(s) in the liver and is then transported by red blood cells (RBCs) to the lung, where it initiates endothelial injury. While several lines of evidence support the requirement of hepatic metabolism for pneumotoxicity, the mechanism and relative importance of RBC transport remain undetermined. The endothelial injury does not appear to be acute cell death but rather a delayed functional alteration that leads to disease of the pulmonary arterial walls by unknown mechanisms. The selectivity of MCT for the lung, as opposed to that of other primarily hepatotoxic PAs, appears likely to be a consequence of the differences in hepatic metabolism and blood kinetics of MCT. A likely candidate for a reactive metabolite of MCT is the dehydrogenation product monocrotaline pyrrole (MCTP). Secondary or phase II metabolism of MCT through glutathione (GSH) conjugation has been characterized recently and appears to represent a detoxification pathway. The role of inflammation in the progression of MCT-induced pulmonary vascular disease is uncertain. Both perivascular inflammation and platelet activation have been proposed as processes contributing to the response of the vascular media. This review presents the experimental evidence supporting these hypotheses and outlines additional questions that arise from them.