Mechanisms and pathology of monocrotaline pulmonary toxicity

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.

Microarray analysis reveals pivotal divergent mRNA expression profiles early in the development of either compensated ventricular hypertrophy or heart failure

Myocardial right ventricular (RV) hypertrophy due to pulmonary hypertension is aimed at normalizing ventricular wall stress. Depending on the degree of pressure overload, RV hypertrophy may progress to a state of impaired contractile function and heart failure, but this cannot be discerned during the early stages of ventricular remodeling. We tested whether critical differences in gene expression profiles exist between ventricles before the ultimate development of either a compensated or decompensated hypertrophic phenotype. Both phenotypes were selectively induced in Wistar rats by a single subcutaneous injection of either a low or a high dose of the pyrrolizidine alkaloid monocrotaline (MCT). Spotted oligonucleotide microarrays were used to investigate pressure-dependent cardiac gene expression profiles at 2 wk after the MCT injections, between control rats and rats that would ultimately develop either compensated or decompensated hypertrophy. Clustering of significantly regulated genes revealed specific expression profiles for each group, although the degree of hypertrophy was still similar in both. The ventricles destined to progress to failure showed activation of pro-apoptotic pathways, particularly related to mitochondria, whereas the group developing compensated hypertrophy showed blocked pro-death effector signaling via p38-MAPK, through upregulation of MAPK phosphatase-1. In summary, we show that, already at an early time point, pivotal differences in gene expression exist between ventricles that will ultimately develop either a compensated or a decompensated phenotype, depending on the degree of pressure overload. These data reveal genes that may provide markers for the early prediction of clinical outcome as well as potential targets for early intervention.

Specific inhibition of p38 mitogen-activated protein kinase with FR167653 attenuates vascular proliferation in monocrotaline-induced pulmonary hypertension in rats

OBJECTIVES

p38 mitogen-activated protein kinase is associated with many clinical entities characterized by inflammation. We postulated that inhibition of p38 mitogen-activated protein kinase with FR167653 attenuates inflammation and the development of pulmonary hypertension in monocrotaline-treated rats.

METHODS

Rats were divided into 4 groups: (1) the control group (daily 0.9% saline), (2) the FR group (daily FR167653, 2 mg . kg(-1) . d(-1)), (3) the MCT group (daily 0.9% saline the day after a single monocrotaline dose, 60 mg/kg), and (4) the MCT+FR group (daily FR167653, 2 mg . kg(-1) . d(-1), the day after a single MCT dose). Body weight, pulmonary artery pressure, and morphometric changes of the pulmonary artery with the histopathologic method were observed weekly for 4 weeks. Also, p38 mitogen-activated protein kinase activity and inflammatory cytokine expression in the lung were measured.

RESULTS

Four weeks after monocrotaline administration, mean pulmonary artery pressure in the MCT+FR group was lower than in the MCT group (MCT+FR vs MCT: 24.7 +/- 1.9 vs 36.5 +/- 2.1 mm Hg; P < .05). In morphometric analysis the percentage of medial wall thickness and the percentage of muscularization in the MCT+FR group were reduced compared with those in the MCT group after 4 weeks (P < .05); however, the number of macrophages was not significantly different. p38 mitogen-activated protein kinase activity was significantly attenuated in the MCT+FR group compared with in the MCT group (7.2 +/- 0.52 vs 2.1 +/- 0.23 fold-increase, P < .05, at 1 week). Although mRNA levels of tumor necrosis factor alpha and interleukin 1beta were reduced in the MCT+FR group compared with in the MCT group (tumor necrosis factor alpha: 1.18 +/- 0.36 vs 3.05 +/- 1.12 fold-increase, P < .05, at 2 weeks; interleukin 1beta: 2.2 +/- 0.34 vs 4.4 +/- 1.09 fold-increase, P < .05, at 1 week), FR167653 did not suppress increased monocyte chemotactic protein 1 mRNA expression induced by monocrotaline (3.2 +/- 0.62 vs 3.1 +/- 0.42 fold-increase, at 1 week).

CONCLUSION

FR167653 significantly attenuates the expression of inflammatory cytokines, ultimately preventing the progression of pulmonary hypertension. These results suggest that p38 mitogen-activated protein kinase might play a central role in the molecular events that underlie the development and progression of pulmonary hypertension.

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.

Chronic sildenafil treatment inhibits monocrotaline-induced pulmonary hypertension in rats

Sildenafil, a phosphodiesterase 5 inhibitor, is currently under investigation for therapy of pulmonary hypertension. This study was designed to investigate chronic effects of sildenafil in monocrotaline (MCT)-induced pulmonary hypertension in rats. Four weeks after a single subcutaneous injection of MCT, the animals displayed nearly threefold elevated pulmonary artery pressure and vascular resistance values, with a concomitant decline in central venous oxygen saturation and arterial oxygenation. Marked right heart hypertrophy was evident, and massive thickening of the precapillary artery smooth muscle layer was histologically apparent. Further deterioration of pulmonary hypertension occurred 6 weeks after MCT injection, with some animals dying during this period because of right heart failure. When chronically administered from Days 14-28, sildenafil significantly increased plasma cyclic guanosine monophosphate and inhibited the development of pulmonary hypertension and right heart hypertrophy, with preservation of gas exchange and systemic arterial pressure. A corresponding efficacy profile was also noted for long-term feeding with sildenafil from Days 28-42. Moreover, the death rate significantly decreased in those animals treated with sildenafil. We conclude that sildenafil attenuates MCT-induced pulmonary hypertension and cor pulmonale in rats.

Rhythmical contractions in pulmonary arteries of monocrotaline-induced pulmonary hypertensive rats

Rhythmical contractions accompanied by an increase in cytosolic Ca2+ concentrations were produced in ring preparations of endothelium-denuded pulmonary arteries from monocrotaline-treated rats, but not in those from vehicle-treated rats, 2-3 h after a resting tension of 15 mN (150-180% of the initial wall length of the artery) was applied. The rhythmical contractions were abolished by nicardipine and ryanodine. Cyclopiazonic acid reduced the relaxation phase of the rhythmical contractions, finally leading to a sustained contraction. Similarly, apamin caused a sustained contraction, whereas charybdotoxin increased the amplitude of the rhythmical contractions. Glibenclamide had no apparent effects on them. Indomethacin and the prostaglandin H2/thromboxane A2 receptor antagonist SQ29548 abolished the rhythmical contractions and reduced the tension, but the thromboxane synthase inhibitor ozagrel had no effect. These results suggest that optimal stretch induces rhythmical contractions in the pulmonary arteries of monocrotaline-induced pulmonary hypertensive rats, to which both Ca2+ influx through voltage-operated Ca2+ channels and Ca2+ release from the endoplasmic reticulum seem to contribute. It is also suggested that small-conductance Ca(2+)-activated K+ channels participate in the relaxation phase of rhythmical contractions. Furthermore, prostaglandin H2 released from nonendothelial cells is likely to play a pivotal role in the induction of rhythmical contractions.

Myocardial force development and structural changes associated with monocrotaline induced cardiac hypertrophy and heart failure

In this study alterations are characterized which occur, in myocardial force development morphological appearance and protein composition, during the development of cardiac hypertrophy and heart failure in monocrotaline (MCT) treated rats. The transition from cardiac hypertrophy to heart failure was studied by comparing the results from control (CON) and two MCT groups (40 and 44 mg/kg body weight). The three experimental groups consisted of at least five animals each. Parameters studied were: body weight (measured daily), lung/body weight ratio, right ventricular wall volume and thickness, and force development in thin right ventricular trabeculae at 27 degrees C, using different extracellular calcium concentrations and pacing frequencies. MCT injection resulted in marked right ventricular hypertrophy and heart failure as evidenced by an up to 2-fold increase in lung/body weight ratio and a 1.7-fold increase in wall volume. The MCT groups showed a negative force-frequency relation and maximum force was up to 2-fold less than in the CON group. Protein analysis by means of one- and two-dimensional gel electrophoresis revealed a marked (7-fold) up-regulation of the slow myosin heavy chain isoform as well as a 4.5-fold increase in the content of the cytoskeletal protein desmin, whereas the mitochondrial protein ATP-synthase content was reduced. Hence MCT-induced cardiac hypertrophy and heart failure result in altered cellular calcium handling, depression of maximum force output, an increase in the economy of myocardial contraction and changes in cytoskeletal structure and energy supply.

A search for an animal model of the Spanish toxic oil syndrome

To date, pathology characteristics of toxic oil syndrome (TOS), a disease associated with consumption of a contaminated cooking oil in Spain in 1981, have not been reproduced in an animal model. As vasculitis, eosinophilia, and a rise in circulating IgE levels were features of the acute phase of TOS, leading to an autoimmune outcome, a review of predisposition to these aspects across species was conducted. The intent was to determine predisposed strains or species that potentially might be effective in testing the toxic oils and thus defining the precise identity of the toxic contaminant(s). A number of potential candidates emerge from this review. Among mice, these include the NZB mouse hybrids, the MRL/lpr and SJL/J strains, and a transgenic mouse model of eosinophilia. The Brown Norway may be the most appropriate rat strain, while beagle dogs inbred to be genetically predisposed to immune complex disease and vasculitis are also a candidate species. Of the more exotic species, the mink and ferret have characteristics that might make them suitable candidates for testing oil samples.

Effect of a serotonin receptor antagonist on interleukin-6-induced pulmonary hypertension in rats

STUDY OBJECTIVES

To determine the significance of serotonin in the pathogenesis of interleukin (IL) 6-induced pulmonary hypertension (IL-6-PH) in rats, the plasma serotonin concentrations, and the effects of a specific antagonist of the serotonin (5-hydroxytryptamine [5-HT]) receptor, 1-[o-(m-methoxyphenyl)ethyl]phenoxy]-3-(dimethylamino)-2-propyl hydrogen succinate hydrochloride (MCI) on the degree of pulmonary hypertension (PH) were investigated in MCI-treated IL-6-PH (IL-6-MCI-PH) rats.

MEASUREMENTS

The thickness of the media of small pulmonary arteries and the ratio of the weight of the right ventricle free wall (RV) to that of the left ventricle with the septum (LV + S) were measured as indexes of the degree of PH. Serotonin concentrations in plasma and in supernatants of cultured vascular smooth muscle cells (VSMCs) stimulated by IL-6 were measured by high-performance liquid chromatography. The amplification of DNA encoding the 5-HT receptor in the lung specimen and VSMCs was performed by polymerase chain reaction.

RESULTS

The degree of PH, as determined by the medial thickness of small pulmonary arteries, was significantly increased in IL-6-PH rats as compared with normal control rats (p<0.05), and that in IL-6-MCI-PH rats was not significantly different from that in normal control rats. The RV/LV + S weight ratio in the IL-6-PH rats was significantly higher than that in normal control rats (p < 0.01). The RV/LV + S weight ratio in IL-6-MCI-PH rats was significantly lower than that in IL-6-PH rats (p < 0.01) and was not significantly different from that in normal control rats. The serotonin concentration was significantly higher in IL-6-PH rats than in normal control rats (p < 0.02), and the serotonin concentration in IL-6-MCI-PH rats was not significantly different from that in the normal control rats. The expression levels of the 5-HT receptor messenger RNA in the lung tissue tended to increase in IL-6-PH rats but was suppressed in IL-6-MCI-PH rats. IL-6 significantly increased the amount of serotonin released from VSMCs (p < 0.02). The expression of the 5-HT receptor messenger RNA was observed with IL-6 stimulation as was observed with serotonin stimulation in VSMCs.

CONCLUSIONS

Serotonin receptor antagonists could be considered as potentially useful agents for the treatment of chronic thromboembolic PH, as well as for that of primary PH and PH associated with collagen vascular diseases.

The cardiac beta-adrenoceptor-G-protein(s)-adenylyl cyclase system in monocrotaline-treated rats

In rats, injection of the alkaloid monocrotaline (MCT) causes right ventricular hypertrophy and cardiac failure. In order to study whether, in MCT-treated rats, changes in the cardiac beta -adrenoceptor-G-protein(s)-adenylyl cyclase system might be comparable to those found in human primary pulmonary hypertension, we assessed in right and left ventricles from MCT-treated rats the components of the beta -adrenoceptor system: the receptor number and subtype distribution (by (-)-[(125)I]iodocyanopindolol binding), the G-proteins (by quantitative Western blotting), and the activity of adenylyl cyclase. A single injection of 60 mg/kg i.p. MCT caused in rats right ventricular hypertrophy (RVH); part of the rats developed cardiac failure (RVF). In these rats the cardiac beta -adrenoceptor-G-protein(s)-adenylyl cyclase system was markedly changed beta -adrenoceptors were desensitized due to a decrease in receptor number, an uncoupling of the receptor from the G(s)-adenylyl cyclase system, a decrease in G(s)and a decrease in the activity of the catalytic unit of adenylyl cyclase. In general, these changes were more pronounced in right ventricles v left ventricles, and in rats with RVF v rats with RVH. On the other hand, cardiac muscarinic receptors and G(i)appeared not to be altered. We conclude that in MCT-treated rats changes in the cardiac beta -adrenoceptor-G-protein(s)-adenylyl cyclase system occur that resemble those observed in human primary pulmonary hypertension. Thus, MCT-treated rat appears to be a suitable animal model to study in more detail the pathophysiology of the development of right heart failure, and to identify new therapeutic possibilities.

Enhanced expression and activity of xanthine oxidoreductase in the failing heart

The molecular basis for heart failure is unknown, but oxidative stress is associated with the pathogenesis of the disease. We tested the hypothesis that the activity of xanthine oxidoreductase (XOR), a free-radical generating enzyme, increases in hypertrophied and failing heart. We studied XOR in two rat models: (1) The monocrotaline-induced right ventricular hypertrophy and failure model; (2) coronary artery ligation induced heart failure, with left ventricular failure and compensatory right ventricular hypertrophy at different stages at 3 and 8 weeks post-infarction, respectively. XOR activity was measured at 30 degrees C and the reaction products were analysed by HPLC. In both models XOR activity in hypertrophic and control ventricles was similar. In the monocrotaline model, the hearts showed enhanced XOR activity in the failing right ventricle (65+/-5 mU/g w/w), as compared to that in the unaffected left ventricle (47+/-3 mU/g P<0.05, n=6-7). In the coronary ligation model, XOR activities did not differ at 3 and 8 weeks. In the infarcted left ventricle, XOR activity increased from 29.4+/-1.4 mU/g (n=6) in sham-operated rats, to 48+/-3 and 80+/-6 mU/g (n=8 P<0.05 v sham) in the viable and infarcted parts of failing rat hearts, respectively. With affinity-purified polyclonal antibody, XOR was localized in CD68+ inflammatory cells of which the number increased more in the failing than in sham-operated hearts. Our results show that the expression of functional XOR is elevated in failing but not in hypertrophic ventricles, suggesting its potential role in the transition from cardiac hypertrophy into failure.

Protein targets of monocrotaline pyrrole in pulmonary artery endothelial cells

A single administration of monocrotaline to rats results in pathologic alterations in the lung and heart similar to human pulmonary hypertension. In order to produce these lesions, monocrotaline is oxidized to monocrotaline pyrrole in the liver followed by hematogenous transport to the lung where it injures pulmonary endothelium. In this study, we determined specific endothelial targets for (14)C-monocrotaline pyrrole using two-dimensional gel electrophoresis and autoradiographic detection of protein metabolite adducts. Selective labeling of specific proteins was observed. Labeled proteins were digested with trypsin, and the resulting peptides were analyzed using matrix-assisted laser desorption ionization mass spectrometry. The results were searched against sequence data bases to identify the adducted proteins. Five abundant adducted proteins were identified as galectin-1, protein-disulfide isomerase, probable protein-disulfide isomerase (ER60), beta- or gamma-cytoplasmic actin, and cytoskeletal tropomyosin (TM30-NM). With the exception of actin, the proteins identified in this study have never been identified as potential targets for pyrroles, and the majority of these proteins have either received no or minimal attention as targets for other electrophilic compounds. The known functions of these proteins are discussed in terms of their potential for explaining the pulmonary toxicity of monocrotaline.

DNA damage cell checkpoint activities are altered in monocrotaline pyrrole-induced cell cycle arrest in human pulmonary artery endothelial cells

Monocrotaline pyrrole (MCTP) causes cyto- and karyomegaly and persistent cell cycle arrest in the G2 stage of the cell cycle in cultured bovine pulmonary artery endothelial cells. To better characterize the cell cycle regulatory mechanisms of this process as well as determine whether this process would occur in cells of human origin, we treated human pulmonary artery endothelial cell (HPAEC) cultures with MCTP and determined, by flow cytometry, the expression of cyclin B1 and p53 in conjunction with DNA content. We also validated by Western blots that the persistence of cdc2 in its inactivated phosphorylated state, previously described in bovine cell cultures, occurred in HPAEC. Alterations in p53, cyclin A, cyclin B1, and cdc25c expression were also examined in Western blots of treated HPAEC extracts. The response of HPAEC to MCTP was compared with that of adriamycin and nocodazole, agents known to cause cell cycle alterations. Results of these experiments demonstrate that HPAEC treated with MCTP develop a population of cells in G2 that has increased cyclin B1 expression. These cells express increased amounts of cdc2 but not cdc25c. The ratio of inactive triphosphorylated cdc2 to the active monophosphorylated form increased moderately from control cultures in contrast to predominance of the active form in nocodazole-treated cultures. In addition, a second population of cells expressing cyclin B1 had continued incorporation of BrdU and DNA content consistent with 8 N chromosomes. A similar 8 N cell population was evident in nocodazole-treated cells but these cells had both cyclin B1 positive and negative components. Compared with adriamycin, a known inducer of p53, MCTP-treated HPAEC expressed p53 only at high concentrations and p53 expression was not coordinated with G2 arrest or polyploidy. We conclude that HPAEC treated with low concentrations of MCTP develop G2 arrest in association with persistent cyclin B1 expression, failure to completely activate cdc2, and continued DNA synthesis through a pathway that is unrelated to altered expression of p53.

Support of sinusoidal endothelial cell glutathione prevents hepatic veno-occlusive disease in the rat

Depletion of sinusoidal endothelial cell glutathione (GSH) has been proposed as a common mechanism leading to hepatic veno-occlusive disease (HVOD). This study examines whether intraportal infusion of GSH can prevent HVOD in the monocrotaline rat model. HVOD was induced in rats with monocrotaline 160 mg/kg i.g. on day 0. GSH was infused intraportally by mini-osmotic pump. Monocrotaline decreased GSH in sinusoidal endothelial cells, but not in liver homogenate. Infusion of GSH, 2 micromol/hr starting day - 1, prevented the decrease in sinusoidal endothelial cell GSH and protected against histological and clinical evidence of HVOD. Protection by GSH was dose-dependent (0.5-2 micromol/hr). In rats receiving continuous GSH infusion, treatment with buthionine sulfoximine starting day - 2 decreased sinusoidal endothelial cell GSH and attenuated the protective effect of GSH against monocrotaline. GSH infusion starting 24 hours after monocrotaline ("glutathione rescue") offered substantial protection to most rats. N-acetyl-L-cysteine conferred protection, but N-acetyl-D-cysteine (an antioxidant that is not a precursor for GSH) had little or no protective effect, and 4-hydroxy TEMPO, a free radical scavenger, was not protective. Discontinuation of the GSH infusion 5 days after monocrotaline administration led to severe hepatic veno-occlusive disease on day 6. In conclusion, monocrotaline selectively depletes sinusoidal endothelial cell GSH. Intraportal infusion of GSH protects against monocrotaline toxicity, at least partially by maintaining sinusoidal endothelial cell GSH levels. Glutathione infusion started after monocrotaline is partially protective. Monocrotaline induces prolonged changes in the liver that remain suppressed as long as GSH is infused.

Impaired endothelium-dependent relaxation by adrenomedullin in monocrotaline-treated rat arteries

The effects of adrenomedullin were evaluated in isolated vascular rings from rats treated with monocrotaline (60 mg/kg, s.c.) causing pulmonary hypertension and right ventricular hypertrophy within 3 to 4 weeks. Sham animals (NaCl-treated rats) were used for comparison. The relaxing effects of adrenomedullin (10(-8) M) and acetylcholine (10(-6) M) were determined in thoracic aorta and pulmonary artery rings precontracted with phenylephrine (10(-7) M). In sham animals, adrenomedullin caused significant vasorelaxation of aorta and pulmonary artery although of different amplitude (24 +/- 3% and 40 +/- 2%, respectively). A greater relaxation was observed in response to acetylcholine. Monocrotaline-treated rats exhibited a reduction in adrenomedullin relaxation in pulmonary artery (54 and 68% loss of effect, at 3 and 4 weeks, respectively, P < 0.01 vs. sham) and comparable reductions in acetylcholine responses. The decrease in adrenomedullin relaxing effect was less pronounced in aorta than in pulmonary artery, suggesting a distinct tissue sensitivity to monocrotaline. In contrast, the relaxing effect of acetylcholine on aorta was decreased at 4 weeks (36% reduction, P < 0.01 vs. sham). In this model, the adrenomedullin-induced relaxation of the pulmonary artery was impaired due to a severe endothelial dysfunction which may contribute partly to the evolving pathophysiological process.

Reversal of impaired calcium homeostasis in the rat diaphragm subjected to Monocrotaline-induced pulmonary hypertension

Monocrotaline (MCT) pneumotoxicity is known to alter the structure of pulmonary vascular wall and impairs endothelial cell function resulting in pulmonary hypertension. Its effect on the diaphragm muscle has not yet been elucidated. This study examines the effect of MCT pneumotoxicity on calcium transport in the rat diaphragm. Pulmonary hypertension induced by MCT pneumotoxicity caused a significant increase (P < 0.001) in calcium accumulation in strips isolated from rat diaphragms. Treatment of rats having received MCT with Indapamide reduced calcium uptake by diaphragmatic strips to levels that are not significantly different from the control (P > 0.05). Treatment with Indapamide alone did not elicit any change in calcium accumulation in the diaphragmatic strips. Treatment of the animals with MCT, Indapamide or both did not produce any significant change (P > 0.05) in the cell volume of the diaphragmatic strips. Pulmonary hypertension increased calcium uptake by the muscle cells in the rat diaphragm which may alter diaphragmatic contractility; an effect that was prevented by Indapamide.

Alteration in lung particle translocation, macrophage function, and microfilament arrangement in monocrotaline-treated rats

Individuals with preexisting cardiopulmonary disease are thought to be more susceptible to acute episodes of particulate pollution resulting in increased morbidity and mortality. Our study was designed to evaluate particle fate and macrophage function in an animal model of monocrotaline (MCT)-induced pulmonary hypertension. Two weeks following a single MCT injection, Sprague-Dawley rats were exposed sequentially to two different colored fluorescent microspheres 1.0 micron in diameter by aerosolization. Morphometric evaluation of lung sections was performed 0 and 24 h following the final particle exposure to determine the intrapulmonary location of inhaled microspheres. A decrease in the number of particles phagocytized by alveolar macrophages and an increase of free particles overlying the epithelium were found in MCT-treated animals compared with control. Pulmonary macrophages recovered by bronchoalveolar lavage were evaluated for chemotactic and phagocytic ability. Macrophage chemotaxis was significantly impaired following MCT treatment compared with controls, whereas phagocytic activity of macrophages lavaged from MCT and control treatment groups was similar. Macrophages were stained for filamentous (F) and globular (G) actin using Texas-Red-labeled phalloidin and Oregon-green-labeled DNase I, respectively. The area of microfilament staining for F and G actin increased, but the ratio of F/G actin was significantly decreased in animals with MCT treatment compared with control. While the responses observed with MCT treatment, such as pulmonary edema, polymorphonuclear leukocytes influx, and unique macrophage morphology may contribute to impaired macrophage function, the change in microfilament arrangement suggests that MCT may inhibit macrophage chemotaxis and impair particle clearance from the lungs.

Prostanoid production in endothelial and Kupffer liver cells from monocrotaline intoxicated rats

A single dose of monocrotaline, a pyrrolizidine alkaloid, was injected into rats in order to produce 25 (Group I) and 45 (Group II) days later a progressive and so called delayed liver injury. The present study investigated the prostanoid production of Kupffer cells and endothelial cells separated from Monocrotaline and saline (Group III) injected rat livers. Kupffer cells: formation of 6 keto Prostaglandin F1 alpha, the major prostacycline metabolite, gradually decreased in Groups I vs II (P < 0.01) and in both Groups I and II vs Controls (P < 0.01). In addition Prostaglandin F2 alpha showed a significant increase in Groups I and II when compared to Group III, (P < 0.001), and Thromboxane B2 was present in both Groups of Monocrotaline treated animals, while it was not detectable in the control Group III. Endothelial cells: 6 keto Prostaglandin F1 alpha decreased in Groups 1 vs II. This differences was significant when compared, and compared to controls (Group III, P < 0.001). Prostaglandin E2 was detected only in Groups I and II. Prostaglandin F2 alpha and Thromboxane B2 could not be detected in any Group. Ultramicroscopy showed morphological cell damage in nonparenchymal cells in Monocrotaline intoxication in Group II, rats sacrificed 45 days after the injection, while it shows normal features in those treated animals sacrificed 25 days after the injection, as well as in control group.

CONCLUSION

A single Monocrotaline injection produces, 25 and 45 days later, severe and progressive alterations in the prostanoid production in Kupffer and Endothelial cells, while ultramicroscopic alterations was only observed 45 days after the injection of Monocrotaline. A decreased production of vasodilators and the presence of vasoconstrictor prostanoids that can participate in the production of the circulatory derangements enhancing liver injury and portal hypertension were also observed.

Chronic pulmonary hypertension--the monocrotaline model and involvement of the hemostatic system

Monocrotaline (MCT) is a toxic pyrrolizidine alkaloid of plant origin. Administration of small doses of MCT or its active metabolite, monocrotaline pyrrole (MCTP), to rats causes delayed and progressive lung injury characterized by pulmonary vascular remodeling, pulmonary hypertension, and compensatory right heart hypertrophy. The lesions induced by MCT(P) administration in rats are similar to those observed in certain chronic pulmonary vascular diseases of people. This review begins with a synopsis of the hemostatic system, emphasizing the role of endothelium since endothelial cell dysfunction likely underlies the pathogenesis of MCT(P)-induced pneumotoxicity. MCT toxicology is discussed, focusing on morphologic, pulmonary mechanical, hemodynamic, and biochemical and molecular alterations that occur after toxicant exposure. Fibrin and platelet thrombosis of the pulmonary microvasculature occurs after administration of MCT(P) to rats, and several investigators have hypothesized that thrombi contribute to the lung injury and pulmonary hypertension. The evidence for involvement of the various components of the hemostatic system in MCT(P)-induced vascular injury and remodeling is reviewed. Current evidence is consistent with involvement of platelets and an altered fibrinolytic system, yet much remains to be learned about specific events and signals in the vascular pathogenesis.

Monocrotaline pyrrole interacts with actin and increases thrombin-mediated permeability in pulmonary artery endothelial cells

One of the earliest morphologic changes evident in the monocrotaline (MCT) model of pulmonary hypertension in rats is microvascular leak. Whether this represents a direct effect of MCT metabolites or is secondary to inflammatory and thrombotic changes remains uncertain. To determine whether MCT directly affects endothelial cell permeability barrier function, we characterized the interaction of the reactive pyrrole intermediate of MCT (MCTP) with endothelial cell actin and characterized its effects on thrombin-mediated signal transduction and monolayer permeability. Bovine pulmonary endothelial cells (BPAEC) treated with MCTP had altered distribution of filamentous actin evident by fluorescence microscopy. Correlative Western blots and autoradiography of actin isolated from BPAEC treated with 14C-MCTP showed comigration of actin and MCTP-derived 14C. MCTP treatment did not alter cellular free Ca2+ concentrations nor did it interfere with thrombin-mediated intracellular Ca2+ signal. Pretreatment with MCTP significantly augmented the thrombin-mediated transudation of Evan's blue albumin in BPAEC monolayers apparently by increasing the size of intercellular gaps. We conclude that MCTP directly interacts with actin to alter its polymerization state but does not significantly affect endothelial cell response to contractile stimulus. Our results suggest that MCTP may affect endothelial cell barrier function through alterations in intracellular junctions.

Monocrotaline pyrrole induces apoptosis in pulmonary artery endothelial cells

In the monocrotaline (MCT) model of pulmonary hypertension, the pulmonary vascular endothelium is the likely early target of the reactive metabolite monocrotaline pyrrole (MCTP). Incubation of cultured bovine pulmonary arterial endothelial cells (BPAEC) with MCTP results in covalent binding to DNA, cell cycle arrest, and delayed but progressive cell death. The mode of cell death in MCTP-induced endothelial damage has not been addressed previously. Since DNA damage is frequently associated with apoptosis, the presence or absence of apoptosis in adherent BPAEC was determined by several techniques, including morphologic and terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling. Two concentrations of MCTP (5 and 34.5 microgram/ml) along with a vehicle control were examined with each assay. Both concentrations of MCTP induced increasing numbers of cells to undergo apoptosis over time beginning as early as 6 h after exposure to MCTP in the high concentration group. Control and vehicle control cells exhibited small amounts of apoptosis (1-2%), which did not change over the duration of the experiment. Additionally, cell membrane integrity was assessed over time by either exposure to membrane-impermeant dyes or measuring LDH release. By either method, BPAEC had increased membrane permeability after about 48 h of either low or high concentration MCTP exposure. We conclude that both a low or high concentration of MCTP causes cell death in BPAEC by inducing apoptosis.

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.

Mice with focal pulmonary fibrosis caused by monocrotaline are insensitive to urethane induction of lung tumorigenesis

To establish the characteristics of an optimized pulmonary fibrosis model, male ICR mice were given 4 weekly sc injections of 150 or 0 mg/kg monocrotaline (MCT) and maintained without further treatment for 33 wk (Experiment 1). The final mortality in the MCT group was 64%. Epithelial cells with large bizarre nuclei and an increased incidence of alveolar/bronchiolar hyperplasias were typically observed. In areas of pulmonary fibrosis, the PCNA labeling index (LI) in the alveolar/airway epithelium was significantly elevated. DNA content analysis demonstrated a larger range (4-8C) for the ploidy pattern of alveolar epithelium with large bizarre nuclei than in the normal epithelium (2C). In Experiment 2, the relationship between pulmonary fibrosis development and lung tumorigenesis was investigated. Mice were given 4 weekly sc injections of 150 and 0 mg/kg MCT, followed by a single i.p. injection of 1,000 or 500 mg/kg urethane (UR) on week 7, then maintained without further treatment for an additional 15 wk. UR following MCT-induced inflammatory changes, fibrosis, and epithelia with large bizarre nuclei but no tumorous lesions, in spite of the fact that treatment with UR alone caused a high incidence of pulmonary tumors. Hyperplasias were seen in all groups, but the multiplicity in the combined groups tended to be decreased by the MCT pretreatment. The present study demonstrated that this new protocol is more suitable than previous one for the experimental production of pulmonary fibrosis. Furthermore, the induction of lung tumors by UR was completely depressed in mice with MCT-induced pulmonary fibrosis, suggesting that alveolar epithelial cells are resistant to this lung carcinogen under these conditions.

Monocrotaline metabolism and distribution in Fisher 344 and Sprague-Dawley rats

The metabolism and distribution of 14C-monocrotaline in Fisher 344 (F344) rats was compared with that in Sprague-Dawley (SD) rats. In vitro microsomal preparations, in situ isolated perfused livers and in vivo excretion and distribution studies were used to discern any differences between these two strains. These strains have previously been shown to differ in their susceptibility to monocrotaline-induced pulmonary hypertension. Hepatic phase I metabolism appears to be similar in both strains with N-oxidation and dehydrogenation to the reactive pyrroles as the major pathways. During the liver perfusions, SD rats generated more monocrotalic acid than F344 rats, but the microsome and excretion studies demonstrated no significant differences in the amount of monocrotalic acid. Monocrotalic acid is a stable byproducer of dehydromonocrotaline reacting with cellular nucleophiles and indicates the amount of monocrotaline dehydrogenation when carboxylesterase activity is negligible. These data suggest that the differences in strain susceptibility to pulmonary vascular toxicity is most likely due to differences in their response to the toxic metabolites.

Pulmonary hemodynamics and blood flow characteristics in chronic pulmonary hypertension

BACKGROUND

Lung transplantation is now an acceptable form of therapy for pulmonary hypertension, but controversy remains regarding the most appropriate surgical procedure. In this study, the changes in pulmonary vascular mechanics occurring in the setting of pulmonary hypertension were investigated using an adult canine model of monocrotaline pyrrole-induced pulmonary hypertension.

METHODS

Animals underwent pulmonary artery catheterization to measure right heart pressures before and 8 weeks after injection of either 3 mg/kg of monocrotaline pyrrole (n = 8) or placebo (n = 8). Eight weeks after injection, hearts underwent instrumentation with an ultrasonic flow probe and micromanometers. Harmonic derivation of functional data was achieved with Fourier analysis.

RESULTS

Significant increases in mean pulmonary artery pressure and pulmonary vascular resistance were observed after monocrotaline pyrrole injection. There was no significant difference in pulmonary blood flow. However, significant increases in input resistance and right ventricular hydraulic power with significant decreases in transpulmonary efficiency were observed.

CONCLUSIONS

Pulmonary hypertension causes significant alterations in pulmonary hemodynamics. Pulmonary blood flow is maintained by a significant increase in total power but with a significant decrease in transpulmonary efficiency. This adult canine model of pulmonary hypertension provides a useful means by which to evaluate surgical options of lung transplantation for improving pulmonary hemodynamics in the setting of chronic pulmonary hypertension.

Endogenous endothelin-1 mediates cardiac hypertrophy and switching of myosin heavy chain gene expression in rat ventricular myocardium

OBJECTIVES

We investigated the role of endogenous endothelin-1 in the development of cardiac hypertrophy in vivo under pressure overload conditions.

BACKGROUND

Endothelin-1, a potent vasoconstrictor peptide, has recently been shown to act as a growth factor of myocardial cells in culture.

METHODS

We examined the effect of an endothelin-A receptor antagonist (FR139317) on the development of right ventricular hypertrophy in rats with monocrotaline-induced pulmonary hypertension. Three groups of rats were studied: those given monocrotaline alone or monocrotaline plus FR139317 and those given vehicle alone (control group).

RESULTS

The ratio of right ventricular systolic pressure to aortic systolic pressure was similarly elevated in rats treated with monocrotaline and monocrotaline plus FR139317. The right ventricular/left ventricular weight ratio was increased in monocrotaline-treated rats but lower in rats treated with monocrotaline plus FR139317 than in those treated with monocrotaline alone (p < 0.01). As a biochemical marker of hypertrophy, the isoform ratio of beta-myosin heavy chain protein was determined for the right ventricular tissue samples. This ratio was increased in all monocrotaline-treated rats but was lower (p < 0.01) in rats given monocrotaline plus FR139317 than in those given monocrotaline alone. The isoform ratio of beta-myosin heavy chain messenger ribonucleic acid quantitated by S1 nuclease mapping also was lower (p < 0.025) in rats receiving monocrotaline plus FR139317 than in those receiving monocrotaline alone.

CONCLUSIONS

These data suggest that blocking the action of endothelin-1 with a receptor antagonist ameliorates cardiac hypertrophy in this model system, and that this action is not mediated by ameliorating hemodynamic changes.

Semotiadil improves survival of rats with monocrotaline-induced pulmonary hypertension: comparison with diltiazem

We compared the effects of semotiadil, a novel Ca2+ channel blocker, with those of diltiazem on survival and regression of right ventricular hypertrophy and media thickening of pulmonary arteries in a rat model of pulmonary hypertension. Pulmonary hypertension was induced by a single injection of monocrotaline (80 mg/kg). Four weeks later, after pulmonary hypertension was confirmed, oral administration of semotiadil (10, 30, or 100 mg/kg/day) or diltiazem (100 or 300 mg/kg/day) was initiated. The rats were observed for 3 weeks. Survival was significantly longer in the group that received semotiadil 100 mg/kg/day than in the groups treated with diltiazem 100 or 300 mg/kg/day. Media thickness and smooth muscle area in pulmonary arteries were significantly less in rats treated with semotiadil 100 mg/kg/day than in animals treated with diltiazem 100 mg/kg/day. The right ventricle to left ventricle mass ratio, right ventricular wall thickness, and right ventricular myocardial fiber diameter were equal in these two groups. Semotiadil 100 mg/kg/day improved the survival of rats, which responded with a significant regression of right ventricular hypertrophy and media thickening of pulmonary arteries in comparison with rats treated with diltiazem 100 or 300 mg/kg/day.

Semotiadil inhibits the development of right ventricular hypertrophy and medial thickening of pulmonary arteries in a rat model of pulmonary hypertension

This study was designed to compare the effects of semotiadil, a novel calcium antagonist, with those of diltiazem on the development of right ventricular hypertrophy and medial thickening of pulmonary arteries in a rat model of pulmonary hypertension. Pulmonary hypertension was induced by a single injection of monocrotaline (80 mg/kg). Twenty-four hours later (day 1), oral administration of semotiadil (10, 30, or 100 mg/kg per day) or diltiazem (100 or 300 mg/kg per day) was initiated. The wall thickness of the right ventricle (RV), the RV myocardial fiber diameter, the percent medial pulmonary artery thickness, and the percent area of smooth muscle in pulmonary arteries were determined on day 28. The magnitude of all parameters was significantly less in the group of seven rats that received semotiadil at 100 mg/kg per day than in the group of seven rats treated with diltiazem at 300 mg/kg per day. Semotiadil at 100 mg/kg per day inhibits the development of RV hypertrophy and medial thickening of pulmonary arteries significantly more effectively than diltiazem at 300 mg/kg per day.

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.

Establishment of an animal model for pulmonary fibrosis in mice using monocrotaline

A preliminary attempt at experimental induction of pulmonary fibrosis in which male ICR mice received 15 weekly sc injections of 200 or 100 mg/kg monocrotaline (MC) revealed that most animals treated with the larger dose died of severe interstitial pneumonia, whereas those given 100 mg/kg exhibited only relatively slight lung injury. Based on these results, male mice were administered sc injections of 200 and 100 mg/kg MC once a week for 9 and 18 times, respectively, and then maintained without any further treatment until week 28 after the start. Mice treated with 200 mg/kg MC showed severe pulmonary damage and died by week 25. Mortalities also occurred in the 100-mg group from week 16, with 11 of 40 animals surviving at the termination of the experiment. Histologically, both dose groups demonstrated severe interstitial pneumonia and/or pulmonary fibrosis. Ultrastructurally, inflammatory edema possibly attributable to injuries of alveolar capillary endothelial cells was observed in the high-dose group at week 8, and there was a remarkable increase in collagen fibers in alveolar septa in this group thereafter. The present study results suggest that lung injuries induced by MC treatment progress to irreversible lung fibrosis and that this animal model may have advantage for studying the pathogenesis of lung cancers in patients with pulmonary fibrosis.

Basic fibroblast growth factor alterations during development of monocrotaline-induced pulmonary hypertension in rats

The chemical signaling pathways which orchestrate lung cell responses in hypertensive pulmonary vascular disease are poorly understood. The present study examined temporal alterations in lung basic Fibroblast Growth Factor (bFGF) in a well characterized rat model of monocrotaline (MCT)-induced pulmonary hypertension. By immunohistochemical analysis, there were progressive increases in bFGF in airway, vascular and gas exchange regions of MCT-treated rat lungs. Increases in bFGF preceded the onset of right ventricular hypertrophy at day 21 after MCT administration. Enhanced bFGF immunostaining was observed as early as day 4 in focal areas of the parenchyma, and by day 14 there was enhanced bFGF staining in alveolar macrophages, neutrophils and alveolar septa, which persisted through day 21. In conducting airways, there was elevated bFGF immunostaining in the smooth muscle cell (SMC) layer by days 4 and 7 and in the ciliated epithelium and its basement membrane at days 14 and 21. Cells morphologically similar to Clara cells in the luminal surfaces of bronchioles stained intensely on days 14 and 21. In the nucleus and cytoplasm of medial SMCs within pulmonary arteries, there was a progressive increase in bFGF staining starting at day 4. Lung bFGF mRNA was increased slightly at days 1, 4 and 7, while lung bFGF protein, as judged by western blot analysis, was increased at days 14 and 21 compared to controls. The present results, considered in teh light of teh documented roles of bFGF in vascular cell migration, growth and synthesis of extracellular matrix components, suggest that bFGF may contribute to the structural remodeling processes underlying the development of chronic pulmonary hypertension in MCT-treated rats.