Improving Topical Skin Delivery of Monocrotaline Via Liposome Gel-based Nanosystems.

BACKGROUND: In this study, a liposomal gel based on a pH-gradient method was used to increase the skin-layer retention of monocrotaline (MCT) for topical administration. METHODS: Using the Box-Behnken design, different formulations were designed to form liposome suspensions with optimal encapsulation efficiency (EE%) and stability factor (KE). In order to keep MCT in liposomes and accumulate in skin slowly and selectively, MCT liposome suspensions were engineered into gels. RESULTS: A pH-gradient method was used to prepare liposome suspensions. The optimal formulation of liposome suspensions (encapsulation efficiency: 83.10 ± 0.21%) was as follows: MCT 12 mg, soybean phosphatidyl choline (sbPC) 200 mg, cholesterol (CH) 41 mg, vitamin E (VE) 5 mg, and citric acid buffer solution (CBS) 4.0 10 mL (pH 7.0). The final formulation of liposomal gels consisted of 32 mL liposome suspensions, 4.76 mL deionized water, 0.40 g Carbopol-940, 1.6 g glycerol, 0.04 g methylparaben, and a suitable amount of triethanolamine for pH value adjustment. The results of in vitro drug release showed that MCT in liposomal gels could be released in 12 h constantly in physiological saline as a Ritger-Peppas model. Compared with plain MCT in gel form, liposomal MCT in gel had higher skin retention in vitro. CONCLUSION: In this study, liposomal gels were formed for greater skin retention of MCT. It is potentially beneficial for reducing toxicities of MCT by topical administration with liposomal gel.

An in vitro study on interaction of anisodine and monocrotaline with organic cation transporters of the SLC22 and SLC47 families.

Current study systematically investigated the interaction of two alkaloids, anisodine and monocrotaline, with organic cation transporter OCT1, 2, 3, MATE1 and MATE2-K by using in vitro stably transfected HEK293 cells. Both anisodine and monocrotaline inhibited the OCTs and MATE transporters. The lowest IC50 was 12.9 µmol·L-1 of anisodine on OCT1 and the highest was 1.8 mmol·L-1 of monocrotaline on OCT2. Anisodine was a substrate of OCT2 (Km = 13.3 ± 2.6 µmol·L-1 and Vmax = 286.8 ± 53.6 pmol/mg protein/min). Monocrotaline was determined to be a substrate of both OCT1 (Km = 109.1 ± 17.8 µmol·L-1, Vmax = 576.5 ± 87.5 pmol/mg protein/min) and OCT2 (Km = 64.7 ± 14.8 µmol·L-1, Vmax = 180.7 ± 22.0 pmol/mg protein/min), other than OCT3 and MATE transporters. The results indicated that OCT2 may be important for renal elimination of anisodine and OCT1 was responsible for monocrotaline uptake into liver. However neither MATE1 nor MATE2-K could facilitate transcellular transport of anisodine and monocrotaline. Accumulation of these drugs in the organs with high OCT1 expression (liver) and OCT2 expression (kidney) may be expected.

Monocrotaline Suppresses RANKL-Induced Osteoclastogenesis In Vitro and Prevents LPS-Induced Bone Loss In Vivo.

BACKGROUND/AIMS: Extensive osteoclast formation plays a critical role in bone diseases, including rheumatoid arthritis, periodontitis and the aseptic loosening of orthopedic implants. Thus, identification of agents that can suppress osteoclast formation and bone resorption is important for the treatment of these diseases. Monocrotaline (Mon), the major bioactive component of crotalaria sessiliflora has been investigated for its anti-cancer activities. However, the effect of Mon on osteoclast formation and osteolysis is not known. METHODS: The bone marrow macrophages (BMMs) were cultured with M-CSF and RANKL followed by Mon treatment. Then the effects of Mon on osteoclast differentiation were evaluated by counting TRAP (+) multinucleated cells. Moreover, effects of Mon on hydroxyapatite resorption activity of mature osteoclast were studied through resorption areas measurement. The involved potential signaling pathways were analyzed by performed Western blotting and quantitative real-time PCR examination. Further, we established a mouse calvarial osteolysis model to measure the osteolysis suppressing effect of Mon in vivo. RESULTS: In this study, we show that Mon can inhibit RANKL-induced osteoclast formation and function in a dose-dependent manner. Mon inhibits the expression of osteoclast marker genes such as tartrate-resistant acid phosphatase (TRAP) and cathepsin K. Furthermore, Mon inhibits RANKL-induced the activation of p38 and JNK. Consistent with in vitro results, Mon exhibits protective effects in an in vivo mouse model of LPS-induced calvarial osteolysis. CONCLUSION: Taken together our data demonstrate that Mon may be a potential prophylactic anti-osteoclastic agent for the treatment of osteolytic diseases caused by excessive osteoclast formation and function.

Pyrrolizidine Alkaloid Secondary Pyrrolic Metabolites Construct Multiple Activation Pathways Leading to DNA Adduct Formation and Potential Liver Tumor Initiation.

Pyrrolizidine alkaloids (PAs) and their N-oxide derivatives are hepatotoxic, genotoxic, and carcinogenic phytochemicals. PAs induce liver tumors through a general genotoxic mechanism mediated by a set of four (±)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5 H-pyrrolizine (DHP)-derived DNA adducts. To date, the primary pyrrolic metabolites dehydro-PAs, their hydrolyzed metabolite DHP, and two secondary pyrrolic metabolites 7-glutathione-DHP (7-GS-DHP) and 7-cysteine-DHP are the known metabolites that can generate these DHP-DNA adducts in vivo and/or in PA-treated cells. Secondary pyrrolic metabolites are formed from the reaction of dehydro-PAs with glutathione, amino acids, and proteins. In this investigation, we determined whether or not more secondary pyrrolic metabolites can bind to calf thymus DNA and to cellular DNA in HepG2 cells resulting in the formation of DHP-DNA adducts using a series of secondary pyrrolic metabolites (including 7-methoxy-DHP, 9-ethoxy-DHP, 9-valine-DHP, 7-GS-DHP, 7-cysteine-DHP, and 7,9-diglutathione-DHP) and synthetic pyrroles for study. We found that (i) many secondary pyrrolic metabolites are DNA reactive and can form DHP-DNA adducts and (ii) multiple activation pathways are involved in producing DHP-DNA adducts associated with PA-induced liver tumor initiation. These results suggest that secondary pyrrolic metabolites play a vital role in the initiation of PA-induced liver tumors.

Liraglutide prevents and reverses monocrotaline-induced pulmonary arterial hypertension by suppressing ET-1 and enhancing eNOS/sGC/PKG pathways.

Liraglutide, a glucagon-like peptide-1 receptor (GLP-1R) agonist, is widely used to treat diabetes. However, its effect on pulmonary arterial hypertension (PAH) is unknown. In this study, we investigated its effects on rats with monocrotaline (MCT)-induced PAH and mechanisms on rat pulmonary artery smooth muscle cells (PASMCs). Liraglutide was investigated for both prevention and treatment of MCT-induced PAH. The hemodynamic and body weight changes, right heart hypertrophy, lung morphology, immune-reactivity of endothelial nitric oxide synthase (eNOS), endothelin-1 and cyclic guanosine monophosphate (cGMP) levels, protein expressions of eNOS, soluble guanylyl cyclase (sGCα), protein kinase G (PKG) and Rho kinase (ROCK) II pathway were measured in both in vivo and in vitro. Cell migration and cell cycle were also determined. Liraglutide both prevented and reversed MCT-induced PAH, right ventricle hypertrophy and pulmonary vascular wall remodeling. Protein expression of ROCK II was increased while eNOS, sGC and PKG were decreased. Pretreatment with liraglutide inhibited platelet-derived growth factor (PDGF)-BB stimulated PASMCs migration, which were associated with cell-cycle arrest at G0/G1 phase. Liraglutide may have both preventive and therapeutic effects on MCT-induced PAH, through the eNOS/sGC/PKG and Rho kinase pathways. Thus, liraglutide may have a therapeutic role in pulmonary vascular remodelling.

Activity of pyrrolizidine alkaloids against biofilm formation and Trichomonas vaginalis.

Crotalaria genus belongs to the subfamily Papilionoideae comprising about 600 species spread throughout tropical, neotropical and subtropical regions. In this study, seeds of Crolatalaria pallida were used to the isolation of usaramine, a pyrrolizidine alkaloid. Thus, Pseudomonas aeruginosa and Staphylococcus epidermidis were utilized as strains to test some activities of this alkaloid, such as antibiofilm and antibacterial. Meanwhile, monocrotaline obtained from Crotalaria retusa seeds, was used as the starting material for synthesis of necine base derivatives with anti-Trichomonas vaginalis potential. Alkaloids were characterized by 1D and 2D NMR techniques and GC-MS analysis. Usaramine demonstrated a highlighted antibiofilm activity against S. epidermidis by reducing more than 50% of biofilm formation without killing the bacteria, thus it could be assumed as a prototype for the development of new antibiofilm molecules for pharmaceutical and industrial purposes. Monocrotaline activity against T. vaginalis was evaluated and results indicated inhibition of 80% on parasite growth at 1mg/mL, in addition, neither cytotoxicity against vaginal epithelial cells nor hemolytic activity were observed. On the other hand, retronecine showed no anti-T. vaginalis activity while azido-retronecine was more active than monocrotaline killing 85% of the parasites at 1mg/mL. In conclusion, pyrrolizidine alkaloids are suggested as promising prototypes for new drugs especially for topical use.

Role of interleukin-1 receptor 1/MyD88 signalling in the development and progression of pulmonary hypertension.

Pulmonary artery smooth muscle cell (PA-SMC) proliferation and inflammation are key components of pulmonary arterial hypertension (PAH). Interleukin (IL)-1ß binds to IL-1 receptor (R)1, thereby recruiting the molecular adaptor myeloid differentiation primary response protein 88 (MyD88) (involved in IL-1R1 and Toll-like receptor signal transduction) and inducing IL-1, IL-6 and tumour necrosis factor-α synthesis through nuclear factor-κB activation.We investigated the IL-1R1/MyD88 pathway in the pathogenesis of pulmonary hypertension.Marked IL-1R1 and MyD88 expression with predominant PA-SMC immunostaining was found in lungs from patients with idiopathic PAH, mice with hypoxia-induced pulmonary hypertension and SM22-5-HTT(+) mice. Elevations in lung IL-1ß, IL-1R1, MyD88 and IL-6 preceded pulmonary hypertension in hypoxic mice. IL-1R1(-/-), MyD88(-/-) and control mice given the IL-1R1 antagonist anakinra were protected similarly against hypoxic pulmonary hypertension and perivascular macrophage recruitment. Anakinra reversed pulmonary hypertension partially in SM22-5-HTT(+) mice and markedly in monocrotaline-treated rats. IL-1ß-mediated stimulation of mouse PA-SMC growth was abolished by anakinra and absent in IL-1R1(-/-) and MyD88(-/-) mice. Gene deletion confined to the myeloid lineage (M.lys-Cre MyD88(fl/fl) mice) decreased pulmonary hypertension severity versus controls, suggesting IL-1ß-mediated effects on PA-SMCs and macrophages. The growth-promoting effect of media conditioned by M1 or M2 macrophages from M.lys-Cre MyD88(fl/fl) mice was attenuated.Pulmonary vessel remodelling and inflammation during pulmonary hypertension require IL-1R1/MyD88 signalling. Targeting the IL-1ß/IL-1R1 pathway may hold promise for treating human PAH.

Priming with ceramide-1 phosphate promotes the therapeutic effect of mesenchymal stem/stromal cells on pulmonary artery hypertension.

Some molecules enriched in damaged organs can contribute to tissue repair by stimulating the mobilization of stem cells. These so-called "priming" factors include bioactive lipids, complement components, and cationic peptides. However, their therapeutic significance remains to be determined. Here, we show that priming of mesenchymal stromal/stem cells (MSCs) with ceramide-1 phosphate (C1P), a bioactive lipid, enhances their therapeutic efficacy in pulmonary artery hypertension (PAH). Human bone marrow (BM)-derived MSCs treated with 100 or 200 µM C1P showed improved migration activity in Transwell assays compared with non-primed MSCs and concomitantly activated MAPK(p42/44) and AKT signaling cascades. Although C1P priming had little effect on cell surface marker phenotypes and the multipotency of MSCs, it potentiated their proliferative, colony-forming unit-fibroblast, and anti-inflammatory activities. In a monocrotaline-induced PAH animal model, a single administration of human MSCs primed with C1P significantly attenuated the PAH-related increase in right ventricular systolic pressure, right ventricular hypertrophy, and thickness of α-smooth muscle actin-positive cells around the vessel wall. Thus, this study shows that C1P priming increases the effects of MSC therapy by enhancing the migratory, self-renewal, and anti-inflammatory activity of MSCs and that MSC therapy optimized with priming protocols might be a promising option for the treatment of PAH patients.

Lysine Adduction by Reactive Metabolite(s) of Monocrotaline.

Pyrrolizidine alkaloids (PAs) are known hepatotoxins. The execution of the toxicities of the alkaloids requires metabolic activation. Protein modification by reactive metabolites of PAs has been suggested to be an important mechanism of the toxic actions of PAs. The objectives of the present study were to define the interactions of dehydromonocrotaline (DHM) with lysine, lysine derivatives, a model peptide, and bovine serum albumin and to explore the lysine modification of hepatic proteins of animals given monocrotaline. DHM was found to react with the ε-amino group of all model compounds tested after incubation with DHM, and the modification reaction preferentially occurred at C7 of the necine base. The lysine residue modification with the same regioselectivity was also observed in hepatic proteins of mice treated with monocrotaline. The observed modification increased with the increase in doses administered to the animals. This work allowed us to better understand the mechanisms of the hepatotoxicity of monocrotaline.

An in vitro comparison of the cytotoxic potential of selected dehydropyrrolizidine alkaloids and some N-oxides.

Plants producing dehydropyrrolizidine alkaloids (DHPAs) are found throughout the world and they are dangerous to human and animal health. Several DHPAs are carcinogenic but only riddelliine has been classified as a potential human carcinogen by the National Toxicology Program. As DHPA-related carcinogenicity is probably linked to cytotoxicity, a model of CRL-2118 chicken hepatocyte cytotoxicity was developed to compare equimolar DHPA exposures between 19 and 300 µM. Alkaloid-related cytotoxicity was estimated using cytomorphology, cell viability reflected by mitochondrial function and cellular degeneration reflected by media lactate dehydrogenase activity. Lasiocarpine induced cytotoxicity and decreased cell viability in a concentration dependent manner at 24 h. At similar concentrations and exposures of 48 and 72 h, seneciphylline, senecionine, monocrotaline and riddelliine were cytotoxic. None of the DHPA-N-oxides were significantly cytotoxic at these concentrations. Using graphic analyses the median cytotoxic concentration (DHPA concentration that produced ½ the maximum response) were estimated. The estimated descending order of cytotoxicity was lasiocarpine, seneciphylline, senecionine, heliotrine, riddelliine, monocrotaline, riddelliine-N-oxide, lycopsamine, intermedine, lasiocarpine-N-oxide and senecionine-N-oxide. This comparison identifies DHPAs that were more cytotoxic than carcinogenic riddelliine. Additional studies to better characterize the carcinogenic potential of these alkaloids are essential to better determine the risk they each may pose for human and animal health.

Reaction of dehydropyrrolizidine alkaloids with valine and hemoglobin.

Pyrrolizidine alkaloid-containing plants are probably the most common poisonous plants affecting livestock, wildlife, and humans. Pyrrolizidine alkaloids exert toxicity through metabolism to dehydropyrrolizidine alkaloids that bind to cellular protein and DNA, leading to hepatotoxicity, genotoxicity, and tumorigenicity. To date, it is not clear how dehydropyrrolizidine alkaloids bind to cellular constituents, including amino acids and proteins, resulting in toxicity. Metabolism of carcinogenic monocrotaline, riddelliine, and heliotrine produces dehydromonocrotaline, dehyroriddelliine, and dehydroheliotrine, respectively, as primary reactive metabolites. In this study, we report that reaction of dehydromonocrotaline with valine generated four highly unstable 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-derived valine (DHP-valine) adducts. For structural elucidation, DHP-valine adducts were derivatized with phenyl isothiocyanate (PITC) to DHP-valine-PITC products. After HPLC separation, their structures were characterized by mass spectrometry, UV-visible spectrophotometry, (1)H NMR, and (1)H-(1)H COSY NMR spectral analysis. Two DHP-valine-PITC adducts, designated as DHP-valine-PITC-1 and DHP-valine-PITC-3, had the amino group of valine linked to the C7 position of the necine base, and the other two DHP-valine-PITC products, DHP-valine-PITC-2 and DHP-valine-PITC-4, linked to the C9 position of the necine base. DHP-valine-PITC-1 was interconvertible with DHP-valine-PITC-3, and DHP-valine-PITC-2 was interconvertible with DHP-valine-PITC-4. Reaction of dehydroriddelliine and dehydroheliotrine with valine provided similar results. However, reaction of valine and dehydroretronecine (DHR) under similar experimental conditions did not produce DHP-valine adducts. Reaction of dehydromonocrotaline with rat hemoglobin followed by derivatization with PITC also generated the same four DHP-valine-PITC adducts. This represents the first full structural elucidation of protein conjugated pyrrolic adducts formed from reaction of dehydropyrrolizidine alkaloids with an amino acid (valine). In addition, it was found that DHP-valine-2 and DHP-valine-4, with the valine amino group linked at the C7 position of the necine base, can lose the valine moiety to form DHP.

[Study on determination method for components in monocrotalinum liposomes and their entrapment efficiency].

To establish an HPLC method for determining components in monocrotalinum liposomes. The results showed a good linear relationship in monocrotalinum liposomes within the concentration range between 1.6-102.4 mg x L(-1) (r = 0.999 8), with RSDs of intra-day precision, inter-day precision, stability and reproducibility of 0.61%, 0.92%, 1.7%, 1.6%, respectively. The recovery rate of monocrotaline was (99.96 +/- 0.50)%. These data indicated that the HPLC method could accurately determine components in monocrotalinum liposomes. Meanwhile, the microcolumn centrifugation method was established to determine the entrapment efficiency of components in monocrotalinum liposomes. As a result, the recovery rate and the blank liposome recovery of free components were (94.44 +/- 0.77)%, (95.86 +/- 0.68 )%, respectively. According to the parallel determination of the entrapment efficiency of three monocrotaline liposomes, their RSD was 4.0%. The data indicated that the microcolumn centrifugation method was an accurate and feasible method for determining the entrapment efficiency of monocrotaline liposomes.

¹H NMR study of monocrotaline and its metabolites in human blood.

Monocrotaline (MCT) is a naturally occurring hepatotoxic pyrrolizidine alkaloid found in plants. This investigation is aimed at furthering the understanding of the role of blood in mediating the transport of MCT and its reactive metabolites in humans. Reactions of monocrotaline and its metabolites, dehydromonocrotaline (DHM), retronecine (RET) and dehydroretronecine (DHR) with human blood plasma, red blood cells (RBCs), and whole blood were studied in vitro by proton nuclear magnetic resonance spectroscopy. In plasma MCT remained intact and weakly associated with plasma proteins, and DHM was rapidly hydrolyzed releasing necic and lactone acids, and the reactive pyrrolic metabolite. MCT and its metabolite DHM were internalized in RBCs to the extent of 46.0% and 48.9% respectively in 30 min. No polymerization of DHR was observed when incubated with plasma and RBCs. The data clearly showed that both human plasma and RBCs could be the carriers for the transportation of MCT and its metabolites, DHM, RET and DHR between organs and could stabilise the reactive MCT metabolite DHR.

Differential cellular responses to protein adducts of naphthoquinone and monocrotaline pyrrole.

Protein-xenobiotic adducts are byproducts of xenobiotic metabolism. While there is a correlation between protein adduction and target organ toxicity, a cause and effect relationship is not often clear. Naphthoquinone (NQ) and monocrotaline pyrrole (MCTP) are two pneumotoxic electrophiles that form covalent adducts with a similar select group of proteins rich in reactive thiols. In this study, we treated human pulmonary artery endothelial cells (HPAEC) with NQ, MCTP, or preformed NQ or MCTP adducts to the protein galectin-1 (gal-1) and examined indicators of reactive oxygen species (ROS) oxidative injury, markers of apoptosis (caspase-3 and annexin V), and gene responses of cellular stress. ROS production was assayed fluorescently using CM-H(2)DCFDA. NQ adducts to gal-1 (NQ-gal) produced 183% more intracellular ROS than gal-1 alone (p < 0.0001). Caspase-3 activity and annexin V staining of phosphatidylserine were used to assess apoptotic activity in treated cells. HPAEC exposed to MCTP-gal had increases in both caspase-3 activation and membrane translocation of annexin V relative to gal-1 alone (p < 0.0001). Direct application of NQ produced significantly more ROS and induced significant caspase-3 activation, whereas MCTP did not. Human bronchial epithelial cells were also exposed to MCTP-gal and found to have significant increases in both caspase-3 activation and annexin V staining in comparison to that of gal-1 (p < 0.05). Western blot analysis showed that both NQ and MCTP significantly induced the Nrf2 mediated stress response pathway despite differences in ROS generation. ER stress was not induced by either adducts or parent compounds as seen by quantitative RT-PCR, but HOX-1 expression was significantly induced by NQ-gal and MCTP alone. Electrophile adduction to gal-1 produces different cytotoxic effects specific to each reactive intermediate.

Retrorsine, but not monocrotaline, is a mechanism-based inactivator of P450 3A4.

Retrorsine (RTS) and monocrotaline (MCT) cause severe toxicities via P450-mediated metabolic activation. The screening of mechanism-based inhibitors showed RTS inactivated 3A4 in the presence of NADPH. Unlike RTS, MCT failed to inhibit P450 3A4 and other enzymes tested. Further studies showed the loss of P450 3A4 activity occurred in a time- and concentration-dependent way, which was not recovered after dialysis. Dextromethorphan, a P450 3A4 substrate, protected the enzyme from the inactivation. Exogenous nucleophile glutathione (GSH) and reactive oxygen species scavengers catalase and superoxide dismutase did not protect P450 3A4 from the inactivation. GSH trapping experiments showed both P450 3A4 and 2C19 converted RTS and MCT to the corresponding electrophilic metabolites which could be trapped by GSH to form 7-GSH-DHP conjugate. We conclude that RTS and MCT are metabolically activated by P450 3A4 and 2C19, and that RTS, but not MCT, is a mechanism-based inactivator of P450 3A4.

Feeding deterrence and detrimental effects of pyrrolizidine alkaloids fed to honey bees (Apis mellifera).

Recent studies have shown the occurrence of plant derived pyrrolizidine alkaloids (PAs) in retail honeys and pollen loads, but little is known about how these compounds influence the fitness of foraging honey bees. In feeding experiments, we tested a mix of tertiary PAs and the corresponding N-oxides from Senecio vernalis, pure monocrotaline, and 1,2-dihydromonocrotaline in 50% (w/w) sucrose solutions. The bees were analyzed chemically to correlate the observed effects to the ingested amount of PAs. PA-N-oxides were deterrent at concentrations >0.2%. 1,2-Unsaturated tertiary PAs were toxic at high concentrations. The observed PAs mortality could be linked directly to the presence of the 1,2-double bond, a well established essential feature of PA cytotoxicity. In contrast, feeding experiments with 1,2-dihydromonocrotaline revealed no toxic effects. Levels of less than 50 microg 1,2-unsaturated tertiary PAs per individual adult bee were tolerated without negative effects. PA-N-oxides fed to bees were reduced partially to the corresponding tertiary PAs. Unlike some specialized insects, bees are not able to actively detoxify PAs through N-oxidation. To gain insight into how PAs are transmitted among bees, we tested for horizontal PA transfer (trophallaxis). Under laboratory conditions, up to 15% of an ingested PA diet was exchanged from bee to bee, disclosing a possible route for incorporation into the honey comb. In the absence of alternative nectar and pollen sources, PA-containing plants might exhibit a threat to vulnerable bee larvae, and this might affect the overall colony fitness.

Human liver microsomal reduction of pyrrolizidine alkaloid N-oxides to form the corresponding carcinogenic parent alkaloid.

Retronecine-based pyrrolizidine alkaloids, such as riddelliine, retrorsine, and monocrotaline, are toxic to domestic livestock and carcinogenic to laboratory rodents. Previous in vitro metabolism studies showed that (+/-)6,7-dihydro-7-hydroxy-1-(hydroxymethyl)-5H-pyrrolizine (DHP) and pyrrolizidine alkaloid N-oxides were the major metabolites of these compounds. DHP is the reactive metabolite of pyrrolizidine alkaloids and pyrrolizidine alkaloid N-oxides are generally regarded as detoxification products. However, a previous study of rat liver microsomal metabolism of riddelliine N-oxide demonstrated that DHP and its parent compound, riddelliine, were generated as the major metabolites of riddelliine N-oxide. In this study the metabolic activation of the three retronecine-based pyrrolizidine alkaloid N-oxides by human liver microsomes is investigated under oxidative and hypoxic conditions. Results shows that both the DHP and the corresponding parent pyrrolizidine alkaloids are the major metabolites of the human liver microsomal metabolism of pyrrolizidine alkaloid N-oxides. Under oxidative conditions, reduction of the N-oxide to pyrrolizidine alkaloid is inhibited and while under hypoxic conditions, DHP formation is dramatically decreased. The oxidative and reductive products generated from the metabolism of pyrrolizidine alkaloid N-oxides are substrate-, enzyme- and time-dependent. In the presence of troleandomycin, a microsomal CYP3A inhibitor, DHP formation is inhibited by more than 70%, while the N-oxide reduction was not affected. The level of microsomal enzyme activity in human liver is comparable with rats. The rate of in vitro metabolism by either human and rat liver microsomes follows the order of riddelliine > or = retrorsine > monocrotaline, and DHP-derived DNA adducts are detected and quantified by 32P-postlabeling/HPLC analysis. Similar DHP-derived DNA adducts are found in liver DNA of F344 rats gavaged with the pyrrolizidine alkaloid N-oxides (1.0 mg/kg). The levels of in vivo DHP-DNA adduct formation is correlated with the level of in vitro DHP formation. Our results indicate that pyrrolizidine alkaloid N-oxides may be hepatocarcinogenic to rats through a genotoxic mechanism via the conversion of the N-oxides to their corresponding parent pyrrolizidine alkaloids, and these results may be relevant to humans.

Dietary alkaloids and the development of androconial organs in Estigmene acrea.

Male salt marsh moths, Estigmene acrea (Lepidoptera: Arctiidae), possess inflatable androconial organs called coremata. Prior to mating males form aggregations and inflate their coremata en masse. The communal display attracts additional males and females for the purpose of mating. The coremata are known to carry the plant-derived dihydropyrrolizine, hydroxydanaidal. This pheromonal substance is derived from secondary plant chemicals called pyrrolizidine alkaloids found in the larval diet. When E. acrea larvae were raised on semi-synthetic diets containing different levels of the pyrrolizidine alkaloid precursors the alkaloids triggered a pronounced morphogenetic effect. Adult males that fed on high levels of the pyrrolizidine alkaloid monocrotaline N-oxide (2500 microg) developed the largest coremata. Males that fed on lower levels of monocrotaline N-oxide (500 microg) or no alkaloid, while normal in body weight, had coremata that were progressively smaller and less robust. The size of the coremata and their commensurate pheromonal charge may have behavioral consequences in the unusual mating system of this species.

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

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

Association of dehydromonocrotaline with rat red blood cells.

The association of radiolabeled monocrotaline pyrrole (DHM) with red blood cell (RBCs) ghosts, globins, and heme was examined to determine their role in the transport and stabilization of this hepatic produced putative toxic metabolite of the pyrrolizidine alkaloid monocrotaline (MCT). Rats were administered 5 mg of DHM/kg, i.v., and RBCs and plasma were harvested at 4 and 24 h. Extensive washing of the RBCs with isotonic phosphate buffer did not decrease the amount of radioactivity associated with the cells. The level of DHM equivalents recovered in the RBCs did not decrease between 4 and 24 h, while the plasma levels, which were 29- and 75-fold lower, respectively, decreased from 5.0 to 2.2 nmol of DHM equiv/g of plasma. Globin chains were found to contain 383 and 453 pmol of DHM equiv/mg of protein, respectively. Rats receiving 10 mg of DHM/kg, i.v., with RBCs collected at 2 h, had approximately double the level of radioactivity associated with their RBCs in addition to 2 times the amount of adducts on the globin chains. Globins and ghosts plus heme (2 h) contained 69% and 2% of the radioactivity, respectively. Globin chains treated with an acidic ethanol solution containing AgNO3 resulted in the removal of 31% of the associated radioactivity. GC/ MS and TLC separation of AgNO3-displaced material revealed the presence of the ethyl ether derivatives of 7-hydroxy-1-(hydroxymethyl)-6,7-dihydro-5H-pyrrolizine. The HPLC separation of globin chains revealed that the majority of radioactivity coeluted with the beta-chains. In conclusion, this study found that the administration of radiolabeled DHM resulted in extensive radioactive labeling of RBCs; similar findings have been reported for [14C]MCT.