Bioactivation of monocrotaline by P-450 3A in rat liver

Monocrotaline induces acutely cerebrovascular lesions, astrogliosis and neuronal degeneration associated with behavior changes in rats: A model of vascular damage in perspective

Pyrrolizidine alkaloids (PAs) are secondary plant metabolites playing an important role as phytotoxins in the plant defense mechanisms and can be present as contaminant in the food of humans and animals. The PA monocrotaline (MCT), one of the major plant derived toxin that affect humans and animals, is present in a high concentration in Crotalaria spp. (Leguminosae) seeds and can induce toxicity after consumption, characterized mainly by hepatotoxicity and pneumotoxicity. However, the effects of the ingestion of MCT in the central nervous system (CNS) are still poorly elucidated. Here we investigated the effects of MCT oral acute administration on the behavior and CNS toxicity in rats. Male adult Wistar were treated with MCT (109 mg/Kg, oral gavage) and three days later the Elevated Pluz Maze test demonstrated that MCT induced an anxiolytic-like effect, without changes in novelty habituation and in operational and spatial memory profiles. Histopathology revealed that the brain of MCT-intoxicated animals presented hyperemic vascular structures in the hippocampus, parahippocampal cortex and neocortex, mild perivascular edema in the neocortex, hemorrhagic focal area in the brain stem, hemorrhage and edema in the thalamus. MCT also induced neurotoxicity in the cortex and hippocampus, as revealed by Fluoro Jade-B and Cresyl Violet staining, as well astrocyte reactivity, revealed by immunocytochemistry for glial fibrillary acidic protein. Additionally, it was demonstrated by RT-qPCR that MCT induced up-regulation on mRNA expression of neuroinflammatory mediator, especially IL1β and CCL2 in the hippocampus and cortex, and down-regulation on mRNA expression of neurotrophins HGDF and BDNF in the cortex. Together, these results demonstrate that the ingestion of MCT induces cerebrovascular lesions and toxicity to neurons that are associated to astroglial cell response and neuroinflammation in the cortex and hippocampus of rats, highlighting CNS damages after acute intoxication, also putting in perspective it uses as a model for cerebrovascular damage.

Skeletal muscle blood flow during exercise is reduced in a rat model of pulmonary hypertension

Pulmonary arterial hypertension (PAH) is characterized by exercise intolerance. Muscle blood flow may be reduced during exercise in PAH; however, this has not been directly measured. Therefore, we investigated blood flow during exercise in a rat model of monocrotaline (MCT)-induced pulmonary hypertension (PH). Male Sprague-Dawley rats (∼200 g) were injected with 60 mg/kg MCT (MCT, n = 23) and vehicle control (saline; CON, n = 16). Maximal rate of oxygen consumption (V̇o2max) and voluntary running were measured before PH induction. Right ventricle (RV) morphology and function were assessed via echocardiography and invasive hemodynamic measures. Treadmill running at 50% V̇o2max was performed by a subgroup of rats (MCT, n = 8; CON, n = 7). Injection of fluorescent microspheres determined muscle blood flow via photo spectroscopy. MCT demonstrated a severe phenotype via RV hypertrophy (Fulton index, 0.61 vs. 0.31; P < 0.001), high RV systolic pressure (51.5 vs. 22.4 mmHg; P < 0.001), and lower V̇o2max (53.2 vs. 71.8 mL·min-1·kg-1; P < 0.0001) compared with CON. Two-way ANOVA revealed exercising skeletal muscle blood flow relative to power output was reduced in MCT compared with CON (P < 0.001), and plasma lactate was increased in MCT (10.8 vs. 4.5 mmol/L; P = 0.002). Significant relationships between skeletal blood flow and blood lactate during exercise were observed for individual muscles (r = -0.58 to -0.74; P < 0.05). No differences in capillarization were identified. Skeletal muscle blood flow is significantly reduced in experimental PH. Reduced blood flow during exercise may be, at least in part, consequent to reduced exercise intensity in PH. This adds further evidence of peripheral muscle dysfunction and exercise intolerance in PAH.

Liquorice Extract and 18β-Glycyrrhetinic Acid Protect Against Experimental Pyrrolizidine Alkaloid-Induced Hepatotoxicity in Rats Through Inhibiting Cytochrome P450-Mediated Metabolic Activation

Misuse of pyrrolizidine alkaloid (PA)-containing plants or consumption of PA-contaminated foodstuffs causes numerous poisoning cases in humans yearly, while effective therapeutic strategies are still limited. PA-induced liver injury was initiated by cytochrome P450 (CYP)-mediated metabolic activation and subsequent formation of adducts with cellular proteins. Liquorice, a hepato-protective herbal medicine, is commonly used concurrently with PA-containing herbs in many compound traditional Chinese medicine formulas, and no PA-poisoning cases have been reported with this combination. The present study aimed to investigate hepato-protective effects of liquorice aqueous extract (EX) and 18β-glycyrrhetinic acid (GA, the primary bioactive constituent of liquorice) against PA-induced hepatotoxicity and the underlying mechanism. Histopathological and biochemical analysis demonstrated that both single- and multiple-treatment of EX (500 mg/kg) or GA (50 mg/kg) significantly attenuated liver damage caused by retrorsine (RTS, a representative hepatotoxic PA). The formation of pyrrole-protein adducts was significantly reduced by single- (30.3% reduction in liver; 50.8% reduction in plasma) and multiple- (32.5% reduction in liver; 56.5% reduction in plasma) treatment of GA in rats. Single- and multiple-treatment of EX also decreased the formation of pyrrole-protein adducts, with 30.2 and 31.1% reduction in rat liver and 51.8 and 53.1% reduction in rat plasma, respectively. In addition, in vitro metabolism assay with rat liver microsomes demonstrated that GA reduced the formation of metabolic activation-derived pyrrole-glutathione conjugate in a dose-dependent manner with the estimated IC₅₀ value of 5.07 µM. Further mechanism study showed that GA inhibited activities of CYPs, especially CYP3A1, the major CYP isoform responsible for the metabolic activation of RTS in rats. Enzymatic kinetic study revealed a competitive inhibition of rat CYP3A1 by GA. In conclusion, our findings demonstrated that both EX and GA exhibited significant hepato-protective effects against RTS-induced hepatotoxicity, mainly through the competitive inhibition of CYP-mediated metabolic activation of RTS.

Protective effects of Dapagliflozin on the vulnerability of ventricular arrhythmia in rats with pulmonary artery hypertension induced by monocrotaline

Monocrotaline (MCT)-induced pulmonary artery hypertension (PAH) has been reported to cause right heart failure (RHF). Moreover, Right heart diseases have been determined to cause ventricular arrhythmia (VA). So we can conclude that MCT-induced PAH increases the incidence of VA. In addition, Previous studies have determined the benefits of Dapagliflozin (DA) on the cardiac system, but the responses of MCT-induced RHF to DA are not fully reported. So the present study sought to evaluate the effects of DA on the MCT-induced PAH. A dose intraperitoneal injection of MCT (60 mg/kg) was carried out to induce a rat model with PAH. DA (60 mg/l) was administered for 4 weeks following MCT injection. Echocardiography, body weight, blood pressure, blood glucose, electrophysiological study, and Western blot were performed. Four weeks after the MCT injection, MCT-treated rats decreased body weight, blood glucose and blood pressure. In addition, MCT caused the formation of PAH and RHF. Moreover, MCT-induced PAH rats increased the incidence of VA, prolonged action potential duration (APD), and shortened effective refractory period (ERP). Additionally, PAH rats significantly prevented the activated expressions of Ion channel proteins such as potassium channel (Kv1.5, Kv2.1, Kv4.2, Kv4.3) and L-type Ca channel (Cav1.2). As we expected, these changes above in PAH rats were reversed when DA was administered. Mechanistically, DA significantly reduced the levels of toll-like receptor (TLR4), the nuclear factor kappa B (NF-κB) in MCT-treated rats. In conclusion, these findings determine that DA reduces the vulnerability of VA in PAH rats through the TLR4/NF-κB signaling pathway.

Right predominant electrical remodeling in a pure model of pulmonary hypertension promotes reentrant arrhythmias

BACKGROUND

Electrophysiological (EP) properties have been studied mainly in the monocrotaline model of pulmonary arterial hypertension (PAH). Findings are confounded by major extrapulmonary toxicities, which preclude the ability to draw definitive conclusions regarding the role of PAH per se in EP remodeling.

OBJECTIVE

The purpose of this study was to investigate the EP substrate and arrhythmic vulnerability of a new model of PAH that avoids extracardiopulmonary toxicities.

METHODS

Sprague-Dawley rats underwent left pneumonectomy (Pn) followed by injection of the vascular endothelial growth factor inhibitor Sugen-5416 (Su/Pn). Five weeks later, cardiac magnetic resonance imaging was performed in vivo, optical action potential (AP) mapping ex vivo, and molecular analyses in vitro.

RESULTS

Su/Pn rats exhibited right ventricular (RV) hypertrophy and were highly prone to pacing-induced ventricular tachycardia/fibrillation (VT/VF). Underlying this susceptibility was disproportionate RV-sided prolongation of AP duration, which promoted formation of right-sided AP alternans at physiological rates. While propagation was impaired at all rates in Su/Pn rats, the extent of conduction slowing was most severe immediately before the emergence of interventricular lines of block and onset of VT/VF. Measurement of the cardiac wavelength revealed a decrease in Su/Pn relative to control. Nav1.5 and total connexin 43 expression was not altered, while connexin 43 phosphorylation was decreased in PAH. Col1a1 and Col3a1 transcripts were upregulated coinciding with myocardial fibrosis. Once generated, VT/VF was sustained by multiple reentrant circuits with a lower frequency of RV activation due to wavebreak formation.

CONCLUSION

In this pure model of PAH, we document RV-predominant remodeling that promotes multiwavelet reentry underlying VT. The Su/Pn model represents a severe form of PAH that allows the study of EP properties without the confounding influence of extrapulmonary toxicity.

Rodent models of hypertension

Elevated blood pressure (BP), or hypertension, is the main risk factor for cardiovascular disease. As a multifactorial and systemic disease that involves multiple organs and systems, hypertension remains a challenging disease to study. Models of hypertension are invaluable to support the discovery of the specific genetic, cellular and molecular mechanisms underlying essential hypertension, as well as to test new possible treatments to lower BP. Rodent models have proven to be an invaluable tool for advancing the field. In this review, we discuss the strengths and weaknesses of rodent models of hypertension through a systems approach. We highlight the ways how target organs and systems including the kidneys, vasculature, the sympathetic nervous system (SNS), immune system and the gut microbiota influence BP in each rodent model. We also discuss often overlooked hypertensive conditions such as pulmonary hypertension and hypertensive-pregnancy disorders, providing an important resource for researchers.

Diurnal hepatic CYP3A11 contributes to chronotoxicity of the pyrrolizidine alkaloid retrorsine in mice

1. Retrorsine (RTS) is a pyrrolizidine alkaloid (distributed in many medicinal plants) that has significant hepatotoxicity. Here, we aimed to determine the daily variations in RTS hepatotoxicity (chronotoxicity) in mice, and to investigate the role of metabolism in generating RTS chronotoxicity.2. Acute toxicity and pharmacokinetic studies were performed with mice after RTS administration at different times of the day. Hepatotoxicity was assessed by measuring plasma ALT (alanine aminotransferase) and AST (aspartate aminotransferase) levels. mRNA and proteins were determined by qPCR and Western blotting, respectively. Time-dependent in vitro metabolism of RTS was assessed by using mouse liver microsomes.3. We found that RTS toxicity was more severe in the dark phase (zeitgeber time 14 or ZT14 and ZT18) than in the light phase (ZT2 and ZT6). This chronotoxicity was associated with a dosing time difference in the systemic exposures of RTS and a pyrrolic ester metabolite (a cause of hepatotoxicity, measured by the levels of pyrrole-GSH conjugate and pyrrole-protein adducts due to a high chemical reactivity). Moreover, the CYP3A11 (a major enzyme for RTS bioactivation) inhibitor ketoconazole decreased the production of pyrrole-GSH conjugate and abrogated diurnal rhythm in RTS metabolism. In addition, E4bp4 (a circadian regulator of Cyp3a11) ablation abolished the rhythm of CYP3A11 expression and abrogated the dosing time-dependency of RTS toxicity.4. In conclusion, RTS chronotoxicity in mice was attributed to time-varying hepatic metabolism regulated by the circadian clock. Our findings have implications for reducing pyrrolizidine alkaloid-induced toxicity via a chronotherapeutic approach.

Impact of human immunodeficiency virus on pulmonary vascular disease

With the advent of anti-retroviral therapy, non-AIDS-related comorbidities have increased in people living with HIV. Among these comorbidities, pulmonary hypertension (PH) is one of the most common causes of morbidity and mortality. Although chronic HIV-1 infection is independently associated with the development of pulmonary arterial hypertension, PH in people living with HIV may also be the outcome of various co-morbidities commonly observed in these individuals including chronic obstructive pulmonary disease, left heart disease and co-infections. In addition, the association of these co-morbidities and other risk factors, such as illicit drug use, can exacerbate the development of pulmonary vascular disease. This review will focus on these complex interactions contributing to PH development and exacerbation in HIV patients. We also examine the interactions of HIV proteins, including Nef, Tat, and gp120 in the pulmonary vasculature and how these proteins alter the endothelial and smooth muscle function by transforming them into susceptible PH phenotype. The review also discusses the available infectious and non-infectious animal models to study HIV-associated PAH, highlighting the advantages and disadvantages of each model, along with their ability to mimic the clinical manifestations of HIV-PAH.

Metabolism-mediated cytotoxicity and genotoxicity of pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) and PA N-oxides are common phytotoxins produced by over 6000 plant species. Humans are frequently exposed to PAs via ingestion of PA-containing herbal products or PA-contaminated foods. PAs require metabolic activation to form pyrrole-protein adducts and pyrrole-DNA adducts which lead to cytotoxicity and genotoxicity. Individual PAs differ in their metabolic activation patterns, which may cause significant difference in toxic potency of different PAs. This review discusses the current knowledge and recent advances of metabolic pathways of different PAs, especially the metabolic activation and metabolism-mediated cytotoxicity and genotoxicity, and the risk evaluation methods of PA exposure. In addition, this review provides perspectives of precision toxicity assessment strategies and biomarker development for the risk control and translational investigations of human intoxication by PAs.

Exercise preconditioning prevents left ventricular dysfunction and remodeling in monocrotaline-induced pulmonary hypertension

Background:

Despite pulmonary arterial hypertension (PAH) directly affects the right ventricle (RV), important structural, functional, and molecular changes also occur in left ventricle (LV). The objective of our study was to analyze the hypothetical cardioprotective effects of exercise preconditioning on LV in rats with monocrotaline (MCT)-induced PAH.

Methods:

Forty male Wistar rats were randomly separated in sedentary (SED) and trained group (EX; running sessions of 60 min/day, 5 days/wk, at 25 m/min, for 4 weeks). After 4 weeks, animals were injected with MCT (60 mg/kg; SED + MCT; EX + MCT) or vehicle (SED + V). Following an additional period of 4 weeks where all animals remained sedentary, we completed LV hemodynamic evaluation in baseline and isovolumic conditions and collected LV samples for histological and molecular analysis.

Results:

Preconditioning with exercise was capable to restore LV systolic and diastolic dysfunction in both baseline and isovolumic conditions (P < .05). This improved was paralleled with prevention of LV cardiomyocytes atrophy, fibrosis, and endothelin 1 mRNA levels (P < .05).

Conclusions:

Our findings suggest that exercise preconditioning can prevent LV dysfunction secondary to MCT-induced PAH, which is of particular interest for the familial form of the disease that is manifested by greater severity or earlier onset.

Sex, Gender, and Sex Hormones in Pulmonary Hypertension and Right Ventricular Failure

Pulmonary hypertension (PH) encompasses a syndrome of diseases that are characterized by elevated pulmonary artery pressure and pulmonary vascular remodeling and that frequently lead to right ventricular (RV) failure and death. Several types of PH exhibit sexually dimorphic features in disease penetrance, presentation, and progression. Most sexually dimorphic features in PH have been described in pulmonary arterial hypertension (PAH), a devastating and progressive pulmonary vasculopathy with a 3-year survival rate <60%. While patient registries show that women are more susceptible to development of PAH, female PAH patients display better RV function and increased survival compared to their male counterparts, a phenomenon referred to as the "estrogen paradox" or "estrogen puzzle" of PAH. Recent advances in the field have demonstrated that multiple sex hormones, receptors, and metabolites play a role in the estrogen puzzle and that the effects of hormone signaling may be time and compartment specific. While the underlying physiological mechanisms are complex, unraveling the estrogen puzzle may reveal novel therapeutic strategies to treat and reverse the effects of PAH/PH. In this article, we (i) review PH classification and pathophysiology; (ii) discuss sex/gender differences observed in patients and animal models; (iii) review sex hormone synthesis and metabolism; (iv) review in detail the scientific literature of sex hormone signaling in PAH/PH, particularly estrogen-, testosterone-, progesterone-, and dehydroepiandrosterone (DHEA)-mediated effects in the pulmonary vasculature and RV; (v) discuss hormone-independent variables contributing to sexually dimorphic disease presentation; and (vi) identify knowledge gaps and pathways forward. © 2020 American Physiological Society. Compr Physiol 10:125-170, 2020.

Involvement of immune responses in pulmonary arterial hypertension; lessons from rodent models

Pulmonary hypertension (PH) is a pathological state with sustained elevation of pulmonary artery (PA) pressure. Since the pathogenesis of PH is mostly irreversible, the disease often comes up with poor prognosis. Pulmonary arterioles are affected by deteriorative changes, such as development of occlusive lesions of thickening of arterial walls. Such processes increase the pulmonary arterial pressure thus lead to consequent injuries such as right ventricle failure. Proliferation, or resistance to apoptosis of pulmonary artery smooth muscle cells (PASMC) and fibroblasts, are characteristic changes observed in the PA in pulmonary arterial hypertension (PAH) patients. PAH can either occur idiopathically or come with other diseases. Emerging evidences suggest that pro-inflammatory processes are closely related to the development of PAH. Therefore, it is inferred that immune cells could be the key factors in PAH development. In this review, we summarize the way how each types of immune cells participate in PAH. We would also like to list the current rodent models used for PAH study.

Structure-dependent hepato-cytotoxic potencies of selected pyrrolizidine alkaloids in primary rat hepatocyte culture

Contamination of food, feed and herbal medicines with plants containing pyrrolizidine alkaloids (PA) leads to measurable amounts of PA in many products. Since a number of PA are hepatotoxic in humans and animals and hepato-carcinogenic in animal experiments, the assessment of the relative toxic potencies of widely occurring PA contaminants warrants detailed investigation. Here, we studied the hepato-cytotoxic potencies of a number of relevant PA congeners in rat hepatocytes in primary culture. It was found that cyclic and open di-esters were much more toxic than mono-esters. Furthermore, the hepatocellular levels of cytochrome P450-catalyzed 7-benzoxyresorufin O-dealkylase (BROD) activity decreasing over time in culture, played an important role for activation of PA into cytotoxic metabolites. With a highly toxic PA (lasiocarpine), inhibition of BROD activity with ketoconazole markedly reduced toxicity while this was not obvious with the less toxic congener lycopsamine. Depletion of cellular glutathione with buthionine sulfoximine had no significant influence on the effects of highly toxic PA whereas it slightly increased toxicity of less potent congeners. Overall, our data partially confirm previously published structure-dependent interim Relative Potency (iREP) factors although for echimidine and monocrotaline in particular, substantial deviations were found, possibly due to specific toxicokinetic properties of these congeners.

Human CYP3A4-mediated toxification of the pyrrolizidine alkaloid lasiocarpine

Pyrrolizidine alkaloids (PA) are widely distributed phytotoxins contaminating food and feed. Hepatic enzymes are considered to bioactivate PA. Previous studies showed differences in the metabolism rate in liver homogenates of different species. Thus, uncertainty remains with respect to the relevance of human metabolism. Our study aimed to analyze whether the PA representative lasiocarpine is toxified by human cytochrome P450 (CYP) enzymes. We compared the metabolic elimination of lasiocarpine in the presence of rat and human S9 fractions and liver microsomes. Experiments with the potent CYP3A/Cyp3a inhibitor ketoconazole and supersomes containing individual human and rat CYPs revealed that enzymes of the CYP3A/Cyp3a family of both species are of major relevance for lasiocarpine metabolism. To assess if metabolism by human CYP3A4 results in a toxification of lasiocarpine we performed experiments with V79 cells. γH2AX and micronucleus formation were analyzed as endpoints for genotoxicity. No effects were observed in the wildtype cells, which lack CYP activity. By contrast, a V79 clone engineered for expression of human CYP3A4 showed concentration-dependent γH2AX and micronucleus formation. Concluding, our results showed the CYP3A4-dependent formation of genotoxic metabolites of lasiocarpine. The results confirm previous data indicating the need to include metabolism of PA for human risk assessment.

Low dose monocrotaline causes a selective pulmonary vascular lesion in male and female pneumonectomized rats

Purpose/Aim: Low doses (30-80 mg/kg) of monocrotaline are commonly used to create experimental models of pulmonary hypertension in rats. At these doses, monocrotaline causes pulmonary endothelial apoptosis and acute lung injury which ultimately results in pulmonary vascular disease. Higher doses of monocrotaline (300 mg/kg) are known to create severe liver injury, but previous investigations with lower doses have not reported histology in other organs to determine whether the vascular injury with monocrotaline is pulmonary-selective or generalized.

MATERIALS AND METHODS

We therefore sought to determine whether monocrotaline caused extra-pulmonary injury at doses commonly used in pulmonary hypertension studies. We performed left pneumonectomy on young male and female rats before administering 50-60 mg/kg monocrotaline 7 days later. We monitored serum chemistry and urine dipsticks during the first 3 weeks while the animals developed pulmonary hypertension. After 3 weeks, we sacrificed animals and stained the lungs and highly vascular visceral organs (kidney, liver, and spleen) for elastin to evaluate the degree of vascular injury and remodeling.

RESULTS

We did not observe proteinuria or significant transaminitis over the 3 weeks following monocrotaline. As previously published, monocrotaline caused severe pulmonary vascular disease with neointimal lesions and medial hypertrophy. We did not identify significant large or small arterial damage in the kidneys, liver, or spleen. Two external veterinary pathologists did not identify histopathology in the kidneys, liver, or spleen of these rats.

CONCLUSIONS

We conclude that 50-60 mg/kg of monocrotaline causes a selective pulmonary vascular lesion and that male and female rats have little non-pulmonary damage over 3 weeks at these doses of monocrotaline.

Hypothesis: Neuroendocrine Mechanisms (Hypothalamus-Growth Hormone-STAT5 Axis) Contribute to Sex Bias in Pulmonary Hypertension

Pulmonary hypertension (PH) is a disease with high morbidity and mortality. The prevalence of idiopathic pulmonary arterial hypertension (IPAH) and hereditary pulmonary arterial hypertension (HPAH) is approximately two- to four-fold higher in women than in men. Paradoxically, there is an opposite male bias in typical rodent models of PH (chronic hypoxia or monocrotaline); in these models, administration of estrogenic compounds (for example, estradiol-17β [E2]) is protective. Further complexities are observed in humans ingesting anorexigens (female bias) and in rodent models, such as after hypoxia plus SU5416/Sugen (little sex bias) or involving serotonin transporter overexpression or dexfenfluramine administration (female bias). These complexities in sex bias in PH remain incompletely understood. We recently discovered that conditional deletion of signal transducer and activator of transcription 5a/b (STAT5a/b) in vascular smooth muscle cells abrogated the male bias in PH in hypoxic mice and that late-stage obliterative lesions in patients of both sexes with IPAH and HPAH showed reduced STAT5a/b, reduced Tyr-P-STAT5 and reduced B-cell lymphoma 6 protein (BCL6). In trying to understand the significance of these observations, we realized that there existed a well-characterized E2-sensitive central neuroendocrine mechanism of sex bias, studied over the last 40 years, that, at its peripheral end, culminated in species-specific male ("pulsatile") versus female ("more continuous") temporal patterns of circulating growth hormone (GH) levels leading to male versus female patterned activation of STAT5a/b in peripheral tissues and thus sex-biased expression of hundreds of genes. In this report, we consider the contribution of this neuroendocrine mechanism (hypothalamus-GH-STAT5) in the generation of sex bias in different PH situations.

STAT5a/b contribute to sex bias in vascular disease: A neuroendocrine perspective

Previous studies have elucidated a neuroendocrine mechanism consisting of the hypothalamus (growth hormone releasing hormone, GHRH) - pituitary (growth hormone, GH) - STAT5a/b axis that underlies sex-biased gene expression in the liver. It is now established that male vs female patterned secretion of GHRH, and thus of circulating GH levels ("pulsatile" vs "more continuous" respectively), leading to differently patterned activation of PY-STAT5a/b in hepatocytes results in sex-biased gene expression of cohorts of hundreds of downstream genes. This review outlines new data in support of a STAT5a/b-based mechanism of sex bias in the vascular disease pulmonary hypertension (PH). Puzzling observations in PH include its 2-4-fold higher prevalence in women but a male-dominance in many rodent models, and, paradoxically, inhibition of PH development by estrogens in such models. We observed that conditional deletion of STAT5a/b in vascular smooth muscle cells (SMC) in mice converted the male-dominant model of chronic hypoxia-induced PH into a female-dominant phenotype. In human idiopathic PH, there was reduced STAT5a/b and PY-STAT5 in cells in late-stage obliterative pulmonary arterial lesions in both men and women. A juxtaposition of the prior liver data with the newer PH-related data drew attention to the hypothalamus-GH-STAT5 axis, which is the major target of estrogens at the level of the hypothalamus. This hypothesis explains many of the puzzling aspects of sex bias in PH in humans and rodent models. The extension of STAT5-anchored mechanisms of sex bias to vascular disease emphasizes the contribution of central neuroendocrine processes in generating sexual dimorphism in different tissues and cell types.

Pyrrolizidine Alkaloids: Potential Role in the Etiology of Cancers, Pulmonary Hypertension, Congenital Anomalies, and Liver Disease

Large outbreaks of acute food-related poisoning, characterized by hepatic sinusoidal obstruction syndrome, hemorrhagic necrosis, and rapid liver failure, occur on a regular basis in some countries. They are caused by 1,2-dehydropyrrolizidine alkaloids contaminating locally grown grain. Similar acute poisoning can also result from deliberate or accidental consumption of 1,2-dehydropyrrolizidine alkaloid-containing herbal medicines, teas, and spices. In recent years, it has been confirmed that there is also significant, low-level dietary exposure to 1,2-dehydropyrrolizidine alkaloids in many countries due to consumption of common foods such as honey, milk, eggs, salads, and meat. The level of 1,2-dehydropyrrolizidine alkaloids in these foods is generally too low and too intermittent to cause acute toxicity. However, these alkaloids are genotoxic and can cause slowly developing chronic diseases such as pulmonary arterial hypertension, cancers, cirrhosis, and congenital anomalies, conditions unlikely to be easily linked with dietary exposure to 1,2-dehydropyrrolizidine alkaloids, especially if clinicians are unaware that such dietary exposure is occurring. This Perspective provides a comprehensive review of the acute and chronic toxicity of 1,2-dehydropyrrolizidine alkaloids and their potential to initiate certain chronic diseases, and suggests some associative considerations or indicators to assist in recognizing specific cases of diseases that may have resulted from dietary exposure to these hazardous natural substances. If it can be established that low-level dietary exposure to 1,2-dehydropyrrolizidine alkaloids is a significant cause of some of these costly and debilitating diseases, then this should lead to initiatives to reduce the level of these alkaloids in the food chain.

A novel ACE2 activator reduces monocrotaline-induced pulmonary hypertension by suppressing the JAK/STAT and TGF-β cascades with restored caveolin-1 expression

INTRODUCTION

Pulmonary hypertension (PH) is characterized by increased pressure in the pulmonary artery and right ventricular hypertrophy (RVH). Recently, angiotensin-converting enzyme 2 (ACE2), which converts angiotensin (Ang) II into Ang-(1-7), was shown to inhibit experimental PH. Here we identified a novel ACE2 activator and investigated how the compound reduced monocrotaline (MCT)-induced PH.

METHODS

To induce PH, Sprague-Dawley rats were injected subcutaneously with MCT, followed by the continuous administration of NCP-2454, an ACE2 activator, using osmotic pumps. Pulmonary arterial compliance was monitored every week until 4 weeks post-injection (wpi). RVH and lung remodeling was evaluated using lung tissue at 4 wpi.

RESULTS

NCP-2454 upregulated the production of Ang-(1-7) when incubated with ACE2 and Ang II. Notably, a continuous infusion of NCP-2454 significantly improved pulmonary arterial compliance, right ventricular systolic pressure, and RVH in MCT-treated rats. Interestingly, NCP-2454 increased the relative expression of ACE2 and MAS mRNA in lung tissue, especially in MCT-treated rats. In addition, the compound inhibited the MCT-induced overexpression of transforming growth factor β, phosphorylation of signal transducer and activator of transcription-3 (STAT3), and interleukin-6 production. The compound also restored the expression of caveolin-1 (Cav-1), which negatively regulates the Janus kinase-STAT signaling cascade.

CONCLUSIONS

NCP-2454 prevented MCT-induced PH by suppressing intracellular inflammatory cascades, an upstream molecular change of which is the disruption of Cav-1 expression.

Progress in solving the sex hormone paradox in pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a devastating and progressive disease with marked morbidity and mortality. Even though being female represents one of the most powerful risk factors for PAH, multiple questions about the underlying mechanisms remain, and two "estrogen paradoxes" in PAH exist. First, it is puzzling why estrogens have been found to be protective in various animal models of PAH, whereas PAH registries uniformly demonstrate a female susceptibility to the disease. Second, despite the pronounced tendency for the disease to develop in women, female PAH patients exhibit better survival than men. Recent mechanistic studies in classical and in novel animal models of PAH, as well as recent studies in PAH patients, have significantly advanced the field. In particular, it is now accepted that estrogen metabolism and receptor signaling, as well as estrogen interactions with key pathways in PAH development, appear to be potent disease modifiers. A better understanding of these interactions may lead to novel PAH therapies. It is the purpose of this review to 1) review sex hormone synthesis, metabolism, and receptor physiology; 2) assess the context in which sex hormones affect PAH pathogenesis; 3) provide a potential explanation for the observed estrogen paradoxes and gender differences in PAH; and 4) identify knowledge gaps and future research opportunities. Because the majority of published studies investigated 17β-estradiol and/or its metabolites, this review will primarily focus on pulmonary vascular and right ventricular effects of estrogens. Data for other sex hormones will be discussed very briefly.

Hepatic cytochrome P450s play a major role in monocrotaline-induced renal toxicity in mice

AIM

Monocrotaline (MCT) in plants of the genus Crotalaria induces significant toxicity in multiple organs including the liver, lung and kidney. Metabolic activation of MCT is required for MCT-induced toxicity. In this study, we attempted to determine whether the toxicity of MCT in kidney was a consequence of the metabolic activation of MCT in the liver.

METHODS

Liver-specific cytochrome P450 reductase-null (Null) mice, wild-type (WT) mice and CYP3A inhibitor ketoconazole-pretreated WT (KET-WT) mice were examined. The mice were injected with MCT (300, 400, or 500 mg/kg, ip), and hepatotoxicity and nephrotoxicity were examined 24 h after MCT treatment. The levels of MCT and its metabolites in the blood, liver, lung, kidney and bile were determined using LC-MS analysis.

RESULTS

Treatment of WT mice with MCT increased the serum levels of alanine aminotransferase, hyaluronic acid, urea nitrogen and creatinine in a dose-dependent manner. Histological examination revealed that MCT (500 mg/kg) caused severe liver injury and moderate kidney injury. In contrast, these pathological abnormalities were absent in Null and KET-WT mice. After injection of MCT (400 and 500 mg/kg), the plasma, liver, kidney and lung of WT mice had significantly lower MCT levels and much higher N-oxide metabolites contents in compared with those of Null and KET-WT mice. Furthermore, WT mice had considerably higher levels of tissue-bound pyrroles and bile GSH-conjugated MCT metabolites compared with Null and KET-WT mice.

CONCLUSION

Cytochrome P450s in mouse liver play a major role in the metabolic activation of MCT and thus contribute to MCT-induced renal toxicity.

The Role of Astrocytes in Metabolism and Neurotoxicity of the Pyrrolizidine Alkaloid Monocrotaline, the Main Toxin of Crotalaria retusa

The metabolic interactions and signaling between neurons and glial cells are necessary for the development and maintenance of brain functions and structures and for neuroprotection, which includes protection from chemical attack. Astrocytes are essential for cerebral detoxification and present an efficient and specific cytochrome P450 enzymatic system. Whilst Crotalaria (Fabaceae, Leguminosae) plants are used in popular medicine, they are considered toxic and can cause damage to livestock and human health problems. Studies in animals have shown cases of poisoning by plants from the genus Crotalaria, which induced damage to the central nervous system. This finding has been attributed to the toxic effects of the pyrrolizidine alkaloid (PA) monocrotaline (MCT). The involvement of P450 enzymatic systems in MCT hepatic and pulmonary metabolism and toxicity has been elucidated, but little is known about the pathways implicated in the bioactivation of these systems and the direct contribution of these systems to brain toxicity. This review will present the main toxicological aspects of the Crotalaria genus that are established in the literature and recent findings describing the mechanisms involved in the neurotoxic effects of MCT, which was extracted from Crotalaria retusa, and its interaction with neurons in isolated astrocytes.

A brief overview of mouse models of pulmonary arterial hypertension: problems and prospects

Many chronic pulmonary diseases are associated with pulmonary hypertension (PH) and pulmonary vascular remodeling, which is a term that continues to be used to describe a wide spectrum of vascular abnormalities. Pulmonary vascular structural changes frequently increase pulmonary vascular resistance, causing PH and right heart failure. Although rat models had been standard models of PH research, in more recent years the availability of genetically engineered mice has made this species attractive for many investigators. Here we review a large amount of data derived from experimental PH reports published since 1996. These studies using wild-type and genetically designed mice illustrate the challenges and opportunities provided by these models. Hemodynamic measurements are difficult to obtain in mice, and right heart failure has not been investigated in mice. Anatomical, cellular, and genetic differences distinguish mice and rats, and pharmacogenomics may explain the degree of PH and the particular mode of pulmonary vascular adaptation and also the response of the right ventricle.

The monocrotaline model of pulmonary hypertension in perspective

Severe forms of pulmonary arterial hypertension (PAH) are characterized by various degrees of remodeling of the pulmonary arterial vessels, which increases the pulmonary vascular resistance and right ventricular afterload, thus contributing to the development of right ventricle dysfunction and failure. Recent years have seen advances in the understanding of the pathobiology of PAH; however, many important questions remain unanswered. Elucidating the pathobiology of PAH continues to be critical to design new effective therapeutic strategies, and appropriate animal models of PAH are necessary to achieve the task. Although the monocrotaline rat model of PAH has contributed to a better understanding of vascular remodeling in pulmonary hypertension, we question the validity of this model as a preclinically relevant model of severe plexogenic PAH. Here we review pertinent publications that either have been forgotten or ignored, and we reexamine the monocrotaline model in the context of human forms of PAH.

Identification of metabolites of adonifoline, a hepatotoxic pyrrolizidine alkaloid, by liquid chromatography/tandem and high-resolution mass spectrometry

Hepatotoxic pyrrolizidine alkaloid (HPA)-containing plants have always been a threat to human and livestock health worldwide. Adonifoline, a main HPA in Senecio scandens Buch.-Ham. ex D. Don (Qianli guang), was used officially as an infusion in cases of oral and pharyngeal infections in China. In this study in vivo metabolism of adonifoline was studied for the first time by identifying the metabolites of adonifoline present in bile, urine and feces of rats using liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS(n)) (ion trap) as well as liquid chromatography/electrospray ionization high-resolution mass spectrometry (LC/ESI-HRMS) (quadrupole-time of flight). In total 19 metabolites were identified and, among them, retronecine-N-oxides were confirmed by matching their fragmentation patterns with their fully characterized synthetic compounds. These metabolites are all involved in both phase I and phase II metabolic processes and the principal in vivo metabolism pathways of adonifoline were proposed.

Intracellular glutathione plays important roles in pyrrolizidine alkaloids-induced growth inhibition on hepatocytes

Pyrrolizidine alkaloids (PAs) are well-known natural hepatotoxins distributed widely in thousands of plants in the world. Adonifoline (Adon), senecionine (Sene) and monocrotaline (Mono) are retronecine-type PAs, and the present study is designed to observe the effects of intracellular glutathione on toxicity of these three PAs in human normal liver L-02 cells. The ratio of cellular reduced glutathione (GSH) and oxidized glutathione (GSSG) was assayed after L-02 cells were incubated with these three PAs for various times. Results showed that Adon, Sene and Mono all significantly decreased the ratio of GSH/GSSG in L-02 cells in the time- and concentration-dependent manner. The results of 3-(4,5-dimethylthiazol-2-yl) 2,5-diphenyltetrazolium bromide (MTT) and trypan blue staining assay showed that these three PAs all significantly decreased cell viability in L-02 cells when pretreated with 10μM BSO (L-Buthionine-S-R-Sulfoximine) for 24h to deplete intracellular GSH. Further results showed that anti-oxidant compounds such as NAC (N-Acetyl-Cysteine) and GSH could rescue the cytotoxicity caused by these three PAs with BSO pretreatment. Taken together, those results suggest that intracellular GSH plays important roles in regulating the cytotoxicity induced by PAs.

Galphaq-protein carboxyl terminus imitation polypeptide (GCIP)-27 inhibits right ventricular hypertrophy induced by monocrotaline in rats

This study was to investigate the probable inhibitory effect of Galphaq-protein carboxyl terminus imitation polypeptide-27 (GCIP-27), the optimized form of GCIP, which is a competition candidate of the activated binding sites on Galphaq, on the right ventricular (RV) hypertrophy induced by monocrotaline (MCT) in rats. We have previously shown that GCIP-27, can prevent the hypertrophyc responses in cultured rat cardiomyocytes induced by noradrenaline and angiotensin II. Male Sprague-Dawley rats were given a single dose (50 mg/kg) of MCT subcutaneouly to induce pulmonary hypertension (PH) and RV hypertrophy. GCIP-27 (30, 90 microg/kg) or vehicle was administered (twice daily, intraperitoneally) from day 1 to day 21. GCIP-27 (90 microg/kg) inhibited the elevated pulmonary arteria systolic pressure (PASP) and mean pulmonary arteria pressure induced by MCT, but its dose at 30 microg/kg only reduced the elevated PASP. And no effect could be seen on the pulmonary arteria diastolic pressure at both two doses. On the other hand, the two doses of GCIP-27 improved significantly the weight ratio of RV to left ventricle plus septum, the RV free wall thickness and pulmonary arteria acceleration time (PAAT). In morphometric observation, GCIP-27 (30, 90g/kg) could attenuate cardiomyocytes hypertrophy, interstitium fibrosis, mitochondria swelling and malformation markedly in RVs of MCT-treated rats. Furthermore, GCIP-27 (30, 90 mug/kg) significantly reduced the overexpression of the proliferating cell nuclear antigen (PCNA) induced by MCT in RV cardiocytes. The results suggest that GCIP-27 can effectively attenuate the RV hypertrophy induced by MCT in rats, which may be mediated by both the direct effect on cardiomyocyte and the secondary effect by reducing PH, and may be involved in its influence on the Gq signal pathway.

Preventive effects of raloxifene, a selective estrogen receptor modulator, on monocrotaline-induced pulmonary hypertension in intact and ovariectomized female rats

We investigated whether the chronic treatment with raloxifene, a selective estrogen receptor modulator, prevents the development of monocrotaline-induced pulmonary hypertension in ovary-intact and ovariectomized female rats. Four weeks after a single subcutaneous injection of monocrotaline (60 mg/kg), right ventricular systolic pressure, right ventricle-to-left ventricle plus septal weight ratio, pulmonary arterial medial thickening and endothelin-1 levels in right ventricular tissue increased significantly in both female rats, compared with saline-treated control rats. These monocrotaline-induced alterations were much greater in ovariectomized rats than the changes in intact females. Daily oral administration of raloxifene (10 mg/kg/day for 4 weeks) significantly attenuated the increase in right ventricular systolic pressure to the same levels in both groups of animals, but raloxifene suppressed the increases in right ventricle-to-left ventricle plus septal weight ratio and pulmonary arterial medial thickness more efficiently in ovariectomized females than the case with intact females. In addition, raloxifene completely suppressed the increase in right ventricular endothelin-1 levels in ovariectomized rats, but not in intact females. These data suggest that chronic treatment with raloxifene effectively prevents the development of monocrotaline-induced pulmonary hypertension in ovariectomized female rats than in intact females, at least in part, by suppressing right ventricular endothelin-1 overproduction.

Hepatocyte transplantation and drug-induced perturbations in liver cell compartments

The potential for organ damage after using drugs or chemicals is a critical issue in medicine. To delineate mechanisms of drug-induced hepatic injury, we used transplanted cells as reporters in dipeptidyl peptidase IV-deficient mice. These mice were given phenytoin and rifampicin for 3 days, after which monocrotaline was given followed 1 day later by intrasplenic transplantation of healthy C57BL/6 mouse hepatocytes. We examined endothelial and hepatic damage by serologic or tissue studies and assessed changes in transplanted cell engraftment and liver repopulation by histochemical staining for dipeptidyl peptidase IV. Monocrotaline caused denudation of the hepatic sinusoidal endothelium and increased serum hyaluronic acid levels, along with superior transplanted cell engraftment. Together, phenytoin, rifampicin, and monocrotaline caused further endothelial damage, reflected by greater improvement in cell engraftment. Phenytoin, rifampicin, and monocrotaline produced injury in hepatocytes that was not apparent after conventional tissue studies. This led to transplanted cell proliferation and extensive liver repopulation over several weeks, which was more efficient in males compared with females, including greater induction by phenytoin and rifampicin of cytochrome P450 3A4 isoform that converts monocrotaline to toxic intermediates. Through this and other possible mechanisms, monocrotaline-induced injury in the endothelial compartment was retargeted to simultaneously involve hepatocytes over the long term. Moreover, after this hepatic injury, native liver cells were more susceptible to additional pro-oxidant injury through thyroid hormone, which accelerated the kinetics of liver repopulation.

CONCLUSION

Transplanted reporter cells will be useful for obtaining insights into homeostatic mechanisms involving liver cell compartments, whereas targeted injury in hepatic endothelial and parenchymal cells with suitable drugs will also help advance liver cell therapy.

Serotonin transporter protein in pulmonary hypertensive rats treated with atorvastatin

HMG-CoA-reductase inhibitors (statins) influence lipid metabolism and have pleiotropic effects. Several statins reduce various forms of pulmonary hypertension (PH) in animal models. The relationship between atorvastatin and expression of serotonin transporter protein (5-HTT) remains unknown. This study focused on the effects of atorvastatin on the course of monocrotaline (MCT)-induced PH and its relation to 5-HTT expression. Male Sprague-Dawley rats were challenged with MCT with or without subsequent daily oral treatment with 0.1, 1, and 10 mg/kg of atorvastatin for 28 days. Over the 4-wk course, the progression of PH was followed by transthoracic echocardiography [pulmonary artery pressure was assessed by pulmonary artery flow acceleration time (PAAT), an estimate reciprocal to pulmonary artery pressure], and, at the end of the 4-wk course, invasive right ventricular pressure, right ventricular weight, quantitative morphology, and 5-HTT expression were measured. MCT caused significant PH as early as 7 days after injection. Atorvastatin treatment increased PAAT and reduced right ventricular pressure, right ventricular hypertrophy, and vascular remodeling over the 4-wk course. MCT challenge was associated with increased pulmonary vascular 5-HTT expression, and atorvastatin treatment reduced the 5-HTT expression. MCT-induced PH over the course of 4 wk can be easily followed by transthoracic echocardiography, and atorvastatin is effective in reducing the PH. Atorvastatin's effects are associated with a decrease of 5-HTT expression.

Decreased left ventricular function, myocarditis, and coronary arteriolar medial thickening following monocrotaline administration in adult rats

Decreased right as well as left ventricular function can be associated with pulmonary hypertension (PH). Numerous investigations have examined cardiac function following induction of pulmonary hypertension with monocrotaline (MCT) assuming that MCT has no direct cardiac effect. We tested this assumption by examining left ventricular function and histology of isolated and perfused hearts from MCT-treated rats. Experiments were performed on 50 male Sprague-Dawley rats [348 +/- 6 g (SD)]. Thirty-seven rats received MCT (50 mg/kg sc; MCT group) while the remainder did not (Control group). Three weeks later, pulmonary artery pressure was assessed echocardiographically in 20 MCT and 8 Control rats. The hearts were then excised and perfused in the constant pressure Langendorff mode to determine peak left ventricular pressure (LVP), the peak instantaneous rate of pressure increase (+dP/dtmax) and decrease (-dP/dtmax), as well as the rate pressure product (RPP). Histological sections were subsequently examined. Pulmonary artery pressure was higher in the MCT-treated group compared with the Control group [12.9 +/- 6 vs. 51 +/- 35.3 mmHg (P < 0.01)]. Left ventricular systolic function and diastolic relaxation were decreased in the MCT group compared with the Control group (+dP/dtmax 4,178 +/- 388 vs. 2,801 +/- 503 mmHg/s, LVP 115 +/- 11 vs. 83 +/- 14 mmHg, RPP 33,688 +/- 1,910 vs. 23,541 +/- 3,858 beats x min(-1) x mmHg(-1), -dP/dtmax -3,036 +/- 247 vs. -2,091 +/- 389 mmHg/s; P < 0.0001). The impairment of cardiac function was associated with myocarditis and coronary arteriolar medial thickening. Similarly depressed ventricular function and inflammatory infiltration was seen in 12 rats 7 days after MCT administration. Our findings appear unrelated to the degree of PH and indicate a direct cardiotoxic effect of MCT.

2-Methoxyestradiol mediates the protective effects of estradiol in monocrotaline-induced pulmonary hypertension

When exposed to chronic hypoxia or toxin monocrotaline (MCT), female animals develop less severe pulmonary arterial hypertension (PH) compared to males; ovariectomy (OVX) exacerbates PH, and OVX animals treated with estradiol (E2) develop less severe disease. There is a line of evidence suggesting that cardiovascular protective effects of E2 are mediated by its major metabolite, 2-methoxyestradiol (2ME). Recently, we have shown that 2ME attenuates the development and retards the progression of MCT-induced pulmonary hypertension in male rats. We hypothesized that the protective effects of E2 in experimental PH are mediated by 2ME. Subsets of intact and OVX female rats were injected saline (Cont and OXV groups) or MCT (60 mg/kg; MCT and OVX-MCT groups) and some of OVX-MCT animals were treated with 2ME (10 microg/kg/h via osmotic minipumps; OVX-MCT+2ME). After 28 days, MCT caused PH, i.e., increased right ventricular peak systolic pressure (RVPSP) and right ventricle/left ventricle+septum (RV/LV+S) ratio, induced inflammatory response in the lungs and caused media hypertrophy (media thickness and % media index) and adventitia widening of small size pulmonary arteries. Ovariectomy exacerbated the disease, i.e., further increased RVPSP, and RV/LV+S ratio, and augmented vascular remodeling and inflammatory response. In diseased OVX rats, treatment with 2ME prevented the worsening of PH and attenuated the inflammatory response and vascular remodeling. No mortality was recorded in the OVX-MCT+2ME group vs. 10% and 36% mortality in the MCT and OVX-MCT group, respectively. This study suggests that 2-methoxyestradiol (a major non-estrogenic metabolite of E2) may mediate the protective effects of estradiol in MCT-induced PH, and warrants further evaluation of 2ME for treatment of PH.

Regulation of Hepatocyte Engraftment and Proliferation after Cytotoxic Drug-Induced Perturbation of the Rat Liver

BACKGROUND

Perturbations in specific liver cell compartments benefit transplanted cell engraftment and/or proliferation. We analyzed whether cytotoxic drugs interfering with the integrity of genomic DNA or cell division could be useful for liver cell transplantation.

METHODS

We used dipeptidyl peptidase IV deficient (DPPIV-) rats as recipients of syngeneic F344 rat hepatocytes. Rats were pretreated with doxorubicin, irinotecan, or vincristine prior to cell transplantation and synergistic liver perturbations were induced by drug administration followed by partial hepatectomy or carbon tetrachloride treatments. Transplanted cells were identified by DPPIV histochemistry and cell engraftment and proliferation were analyzed morphometrically. Perturbations in endothelial, Kupffer cell, and hepatocyte compartments were analyzed by electron microscopy, carbon incorporation, and blood tests, respectively.

RESULTS

Cell engraftment was improved in rats treated with doxorubicin but not with irinotecan or vincristine. Doxorubicin disrupted endothelial cells for up to seven days without causing Kupffer cell or hepatocellular toxicity. Neither doxorubicin nor vincristine induced liver repopulation in animals up to three months, including after partial hepatectomy or carbon tetrachloride-induced additional liver injury.

CONCLUSIONS

Doxorubicin-induced hepatic endothelial damage enhanced cell engraftment, which should be useful in cell therapy strategies.

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.

The coagulation system contributes to synergistic liver injury from exposure to monocrotaline and bacterial lipopolysaccharide

Coexposure to a noninjurious dose of bacterial lipopolysaccharide (LPS; 7.4 x 106 EU/kg) and a nontoxic dose of the food-borne toxin monocrotaline (MCT; 100 mg/kg) leads to synergistic hepatotoxicity in Sprague-Dawley rats. Inflammatory factors, such as Kupffer cells (KCs), tumor necrosis factor-alpha (TNF)-alpha, and neutrophils (polymorphonuclear leukocytes; PMNs), are critical to the pathogenesis. Inasmuch as activation of the coagulation system and sinusoidal endothelial cell (SEC) injury precede hepatic parenchymal cell (HPC) injury, and since fibrin deposition occurs within liver lesions, the coagulation system might be a critical component of injury. In this study, this hypothesis is tested, and the interdependence of the coagulation system and inflammatory factors is explored. Administration of the anticoagulants heparin or warfarin to MCT/LPS-cotreated animals attenuated HPC and SEC injury. Morphometric analysis revealed that anticoagulant treatment significantly reduced the area of centrilobular and midzonal lesions. Heparin treatment also reduced fibrin deposition in these regions. Furthermore, anticoagulant treatment decreased hepatic PMN accumulation but did not affect plasma TNF-alpha concentration. Neither KC inactivation nor TNF-alpha depletion prevented activation of the coagulation system. PMN depletion, however, prevented coagulation system activation, suggesting that PMNs are needed for this response. These results provide evidence that the coagulation system and its interplay with PMNs are important in the pathogenesis of MCT/LPS-induced liver injury.

Gender differences in microsomal metabolic activation of hepatotoxic clivorine in rat

The gender differences in the in vitro microsomal metabolic activation of hepatotoxic clivorine, a representative naturally occurring hepatotoxic otonecine type pyrrolizidine alkaloid, in Sprague-Dawley rats and their relation to the gender differences in susceptibility to clivorine intoxication were reported in the present study. Clivorine-induced liver damage in the male rat via metabolic activation to form the reactive pyrrolic ester followed by covalent binding to liver tissue constituents has been reported previously by our research group. The present study demonstrated, for the first time, that cytochromes p450 3A1 and 3A2, which are constitutively expressed in the male rat, might play a significant role in the metabolic activation of clivorine in the rat. Thus, in the male rat, the metabolic activation by liver microsomes to form the reactive pyrrolic ester was found as the only direct metabolic pathway of clivorine followed by subsequent formation of the toxic tissue-bound pyrroles leading to hepatotoxicity. In the case of the female rat, a less significant metabolic activation was observed, whereas the formations of two novel nonpyrrolic metabolites were determined as the predominant biotransformations. None of the four cDNA-expressed rat enzymes (cytochrome p450 2C12, 2E1, 3A1, 3A2) tested could catalyze the formation of these two new metabolites. Furthermore, the female rat (LD(50) = 114 +/- 9 mg/kg, i.p.) was found to be significantly less susceptible to clivorine intoxication than the male rat (LD(50) = 91 +/- 3 mg/kg, i.p.). Therefore, the results suggested that a significantly lower metabolic activation due to the lack of cytochrome p450 3A1 and p450 3A2 activities mainly accounted for the smaller susceptibility of the female rat to clivorine intoxication.

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

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

Heme oxygenase-1 reduces murine monocrotaline-induced pulmonary inflammatory responses and resultant right ventricular overload

Monocrotaline (MT), a pyrrolizidine alkaloid, causes pulmonary hypertension (PH) in rats and is widely utilized to analyze the pathophysiology of PH. However, a murine PH model with which transgenic animals may be used has not been established. To establish a murine MT-induced PH model, we administered different amounts of MT and determined the extent of right ventricular (RV) overload and PH. We also examined the expression of heme oxygenase-1 (HO-1), a potential antistress protein in MT-treated animals, and evaluated the functional role of HO-1 by administering an HO-1 inhibitor. Significant pulmonary inflammation and RV hypertrophy were observed when mice were given 600 mg/kg weight of MT weekly for 8 weeks. In addition, elevated RV pressure and induction of HO-1 in lung and RV were observed with this dose of MT. Interestingly, inhibition of HO activity promoted inflammatory changes in the lung and the resultant RV hypertrophy. HO-1 may play defensive roles against murine MT-induced pulmonary inflammation and the resultant RV overload.

The temporal relationship between bacterial lipopolysaccharide and monocrotaline exposures influences toxicity: shift in response from hepatotoxicity to nitric oxide-dependent lethality

Liver injury from a variety of hepatotoxicants, including the food-borne phytotoxin monocrotaline (MCT), can be augmented by exposure to a noninjurious dose of the inflammagen bacterial lipopolysaccharide (LPS). In a previous study, a nontoxic dose of LPS given 4 h after MCT resulted in synergistic hepatotoxicity within 12-18 h. This study was designed to determine whether temporal differences in MCT and LPS exposure affect toxicity. When LPS (3.4 x 10(6) EU/kg; iv) was given one hour before MCT (100 mg/kg; ip), hepatotoxicity developed between 4 and 8 h after MCT administration, and mortality was much greater than when LPS was administered 4 h after MCT. To explore this difference, the temporal relationship between LPS and MCT exposure (7.4 x 10(6) EU/kg and 100 mg/kg, respectively) was altered. Twenty-four-hour survival was high in animals that received LPS 4 h before (86%) or after (88%) MCT, but it decreased markedly when LPS was administered 1 h before MCT (17%). Using this latter dosing regimen, animals became moribund as early as 4 h after MCT administration. Since liver injury was similar from regimens that differed greatly in mortality, death appeared to result from extrahepatic causes. To explore a role for nitric oxide (NO)-induced shock in this regimen, animals were treated with aminoguanidine (AG), an inhibitor of inducible NO synthase, prior to administration of LPS given an hour before MCT. In the cotreated animals, AG significantly attenuated mortality and decreased plasma nitrate/nitrite concentrations, markers of NO biosynthesis. Hence, the primary target of toxicity from MCT and LPS cotreatment appeared to shift from the liver to an extrahepatic site or sites as exposure to these agents occurred closer together temporally. NO appears to be causally involved in the deaths of animals treated with LPS 1 h before MCT.

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.

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.

Simultaneous analysis of clivorine and its four microsomal metabolites by high-performance liquid chromatography

A specific high-performance liquid chromatographic assay was developed for a simultaneously qualitative and quantitative determination of clivorine, a hepatotoxic otonecine-type pyrrolizidine alkaloid, and its four putative hepatotoxicity-related metabolites, namely dehydroretronecine, 7-glutathionyldehydroretronecine, 7,9-diglutathionyldehydroretronecine, and clivoric acid, generated in rat microsomal incubation. This simultaneous determination was conducted by a direct analysis of aliquots of the supernatant of incubates using a specific two-column set-up. Impurities in the supernatant were firstly eluted out from the first PRP-1 guard column (50x4.1 mm) during an initial 5 min washing period with isocratic elution by mobile phase A (0.2% formic acid at pH 3.4 adjusted by ammonia). Subsequently, the guard column was then connected to the second PRP-1 analytical column (250x4.6 mm) and analytes were separated by a gradient elution with mobile phases A and B (acetonitrile). The assay provided good reproducibility and accuracy for all analytes tested with less than 12% of overall intra- and inter-day variations and higher than 87% of overall accuracy. This developed method was successfully applied to determine the intact clivorine and its four metabolites generated in rat microsomal incubation.