Influences of various xenobiotic inducers on cytocidal toxicity of lasiocarpine and senecionine in primary cultures of rat hepatocytes

Use of physiologically based kinetic modelling-facilitated reverse dosimetry to convert in vitro cytotoxicity data to predicted in vivo liver toxicity of lasiocarpine and riddelliine in rat

Lasiocarpine and riddelliine are pyrrolizidine alkaloids (PAs) present in food and able to cause liver toxicity. The aim of this study was to investigate whether physiologically based kinetic (PBK) modelling-facilitated reverse dosimetry can adequately translate in vitro concentration-response curves for toxicity of lasiocarpine and riddelliine to in vivo liver toxicity data for the rat. To this purpose, PBK models were developed for lasiocarpine and riddelliine, and predicted blood concentrations were compared to available literature data to evaluate the models. Concentration-response curves obtained from in vitro cytotoxicity assays in primary rat hepatocytes were converted to in vivo dose-response curves from which points of departure (PODs) were derived and that were compared to available literature data on in vivo liver toxicity. The results showed that the predicted PODs fall well within the range of PODs derived from available in vivo toxicity data. To conclude, this study shows the proof-of-principle for a method to predict in vivo liver toxicity for PAs by an alternative testing strategy integrating in vitro cytotoxicity assays with in silico PBK modelling-facilitated reverse dosimetry. The approach may facilitate prediction of acute liver toxicity for the large number of PAs for which in vivo toxicity data are lacking.

In Vitro Metabolism of Isoline, a Pyrrolizidine Alkaloid fromLigularia duciformis, by Rodent Liver Microsomal Esterase and Enhanced Hepatotoxicity by Esterase Inhibitors

Isoline, a major retronecine-type pyrrolizidine alkaloid (PA) from the Chinese medicinal herb Ligularia duciformis, was suggested to be the most toxic known PA. Its in vitro metabolism was thus examined in rat and mouse liver microsomes, and its toxicity was compared with that of clivorine and monocrotaline after i.p. injection in mice. Isoline was more rapidly metabolized by both microsomes than clivorine and monocrotaline and converted to two polar metabolites M1 and M2, which were spectroscopically determined to be bisline (a deacetylated metabolite of isoline) and bisline lactone, respectively. Both metabolites were formed in the presence or absence of an NADPH-generating system with liver microsomes but not cytosol. Their formation was completely inhibited by the esterase inhibitors, triorthocresyl phosphate (TOCP) and phenylmethylsulfonyl fluoride, but not at all or partially by cytochrome P450 (P450) inhibitors, alpha-naphthoflavone and proadifen (SKF 525A), respectively. These results demonstrated that both metabolites were produced by microsomal esterase(s) but not P450 isozymes. The esterase(s) involved showed not only quite different activities but also responses to different inhibitors in rat and mouse liver microsomes, suggesting that different key isozyme(s) or combinations might be responsible for the deacetylation of isoline. Isoline injected i.p. into mice induced liver-specific toxicity that was much greater than that with either clivorine or monocrotaline, as judged by histopathology as well as serum alanine aminotransferase and aspartate aminotransferase levels. Isoline-induced hepatotoxicity was remarkably enhanced by the esterase inhibitor TOCP but was reduced by the P450 inhibitor SKF 525A, indicating that rodent hepatic esterase(s) played a principal role in the detoxification of isoline via rapid deacetylation in vivo.

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.

Model systems for detecting the hepatic toxicity of pyrrolizidine alkaloids and pyrrolizidine alkaloid N-oxides

Indicine N-oxide (INO) is a pyrrolizidine alkaloid (PA) with antitumor activity in animals and humans. Prior studies showed that despite the known hepatic toxicity of the PAs, INO did not produce hepatic toxicity in animals but caused unpredictable lethal hepatic toxicity in humans. In this study we have attempted to find a model system for predicting the hepatotoxic potential of antitumor PAs. Primary cultures of rat hepatocytes showed toxicity only with the most hepatotoxic PAs such as lasiocarpine, but did not detect toxicity with other PAs. Subchronic intraperitoneal administration of PAs to weanling rats and adult mice produced, in surviving animals, hepatic megalocytosis and centrilobular necrosis with heliotrine (H) and 9-O-(R(-)-2-(4'-chlorophenyl)-2-hydroxybutyryl)retronecine N-oxide (RC1NO) but only megalocytosis with INO. Thus, despite previous reports, weanling rats offered no advantage over adult mice for detecting significant hepatic toxicity with PAs. Phenobarbital pretreatment of the mice did not increase the hepatic toxicity of any of the PAs. Subchronic oral administration of PAs to adult mice produced hepatic megalocytosis and centrilobular necrosis in surviving animals with H and RC1NO and megalocytosis with INO. Animals that died acutely following oral administration of INO showed hepatic centrilobular necrosis. Administration of several courses of INO intravenously to dogs produced histological evidence of centrilobular hemorrhagic necrosis. It is concluded that there is no single animal model that will predict hepatic toxicity of the type seen in humans with the antitumor PAs. A combination of studies using adult mice and dogs and lethal doses of the PAs offers the best way of detecting potential hepatic toxicity.

Role of glutathione in the toxicity of the sesquiterpene lactones hymenoxon and helenalin

Hymenoxon and helenalin are toxic sesquiterpene lactones present in the toxic range plants Hymenoxys odorata and Helenium microcephalum. Helenalin (25 mg/kg) or hymenoxon (30 mg/kg) administered to immature male ICR mice caused a rapid decrease in hepatic glutathione levels and were lethally toxic to greater than 60% of the animals within 6 d. L-2-Oxothiazolidine 4-carboxylate (OTC), a compound that elevates cellular glutathione levels, administered to mice 6 or 12 h before either helenalin or hymenoxon protected against hepatic glutathione depletion and the lethal toxicity of these toxins. OTC administered at the same time as the sesquiterpene lactones was not protective, suggesting that the critical events against which glutathione is protective occur within the first 6 h. In primary rat hepatocyte cultures, hymenoxon and helenalin (4-16 microM) caused a rapid lethal injury as determined by the release of lactate dehydrogenase. Cotreatment of cultures with N-acetylcysteine at high concentrations (4 mM) afforded significant protection against lethal injury by both toxins. In contrast, BCNU, which inhibits glutathione reductase, or diethylmaleate, which depletes hepatocellular glutathione, potentiated the hepatotoxicity of helenalin and hymenoxon in monolayer rat hepatocytes. These studies suggest that the in vivo and in vitro toxicity of hymenoxon and helenalin is strongly dependent on hepatic glutathione levels, which hymenoxon and helenalin rapidly deplete at very low concentrations.

Aflatoxin B1 and acetaminophen induce different cytoskeletal responses during prelethal hepatocyte injury

Primary monolayer cultures of adult rat hepatocytes exposed to the hepatocarcinogen aflatoxin B1 (AFB1) undergo a characteristic prelethal cytomorphological change that is distinct from their response to the necrogenic noncarcinogenic hepatotoxin, acetaminophen (AAP). Since changes in cell shape are mediated, at least in part, by the F-actin cytoskeleton, we designed experiments to study early prelethal alterations in the distribution of actin microfilaments in monolayer rat hepatocytes exposed to AFB1 (100 microM) or AAP (16 mM). Using rhodamine-phalloidin fluorescence microscopy, we observed that normal hepatocytes showed a submembranous F-actin distribution with focal short microfilaments extending into filopodia along the periphery of the cell. Hepatocytes exposed to AFB1 for several hours exhibited retraction of their cytoplasm within a prominent circumferential peripheral band of F-actin microfilament bundles. Retraction of focal areas of peripheral cytoplasm was associated with an increased prominence of the radial F-actin-containing filopodia. Subsequently, there appeared peripheral blebs containing very little F-actin. Hepatocytes exposed to equivalently lethal concentrations of AAP initially remained structurally normal. After several hours, the cells exhibited a prominent polar aggregate of short microfilament bundles without the formation of blebs. Both the blebbing and the polar aggregation of F-actin bundles occurred prior to cell death as shown by lactate dehydrogenase release and trypan blue exclusion. These studies support the hypothesis that the lethal effects of these two agents may occur by different biological mechanisms that are associated with remarkably distinct prelethal cytoskeletal responses.

Comparative cytopathology of primary rat hepatocyte cultures exposed to aflatoxin B1, acetaminophen, and other hepatotoxins

The cytomorphological alterations in primary monolayer cultures of Fischer 344 rat hepatocytes exposed continuously to lethal concentrations of various hepatotoxins were compared. Aflatoxin B1 (AFB1) at concentrations of 1 and 10 microM killed 67 and 97% hepatocytes, respectively, at 48 hr, as determined by release of lactate dehydrogenase (LDH) into culture medium, or by trypan blue uptake. AFB1 produced numerous radial finger-like projections (blebs) at attached margins of cytoplasm in almost all hepatocytes between 6 and 24 hr. Formation of blebs was dose dependent and preceded release of LDH and trypan blue uptake. Similar marginal blebs were produced by cycloheximide, 2-acetylaminofluorene, N-hydroxy-2-acetylaminofluorene, and senecionine. By comparison, acetaminophen, at equivalently lethal concentrations (4 or 16 mM) did not cause marginal blebs, but resulted in a similar time course of LDH release and trypan blue uptake. Carbon tetrachloride and bromobenzene also failed to produce blebs at lethal concentrations. Phalloidin and cytochalasin B rapidly produced smaller spherical blebs over the entire surface of all hepatocytes, distinct from the finger-like blebs produced only at the free attached margins of cells exposed to AFB1. Blebs and lethal injury by AFB1 were reduced by phenobarbitone or 3-methylcholanthrene pretreatments and by SKF-525-A in culture. The demonstration of morphological prelethal changes in hepatocytes injured by different classes of hepatotoxins in culture provides a means of differentiating the early biochemical mechanisms by which hepatotoxins and hepatocarcinogens lethally injure hepatocytes before membrane permeability to LDH and trypan blue is detectable.