Active Transport of Hepatotoxic Pyrrolizidine Alkaloids in HepaRG Cells

PBTK model-based analysis of CYP3A4 induction and the toxicokinetics of the pyrrolizidine alkaloid retrorsine in man

Cytochrome P450 (CYP)3A4 induction by drugs and pesticides plays a critical role in the enhancement of pyrrolizidine alkaloid (PA) toxicity as it leads to increased formation of hepatotoxic dehydro-PA metabolites. Addressing the need for a quantitative analysis of this interaction, we developed a physiologically-based toxicokinetic (PBTK) model. Specifically, the model describes the impact of the well-characterized CYP3A4 inducer rifampicin on the kinetics of retrorsine, which is a prototypic PA and contaminant in herbal teas. Based on consumption data, the kinetics after daily intake of retrorsine were simulated with concomitant rifampicin treatment. Strongest impact on retrorsine kinetics (plasma AUC24 and C max reduced to 67% and 74% compared to the rifampicin-free reference) was predicted directly after withdrawal of rifampicin. At this time point, the competitive inhibitory effect of rifampicin stopped, while CYP3A4 induction was still near its maximum. Due to the impacted metabolism kinetics, the cumulative formation of intestinal retrorsine CYP3A4 metabolites increased to 254% (from 10 to 25 nmol), while the cumulative formation of hepatic CYP3A4 metabolites was not affected (57 nmol). Return to baseline PA toxicokinetics was predicted 14 days after stop of a 14-day rifampicin treatment. In conclusion, the PBTK model showed to be a promising tool to assess the dynamic interplay of enzyme induction and toxification pathways.

OCT1-dependent uptake of structurally diverse pyrrolizidine alkaloids in human liver cells is crucial for their genotoxic and cytotoxic effects

Pyrrolizidine alkaloids (PAs) are important plant hepatotoxins, which occur as contaminants in plant-based foods, feeds and phytomedicines. Numerous studies demonstrated that the genotoxicity and cytotoxicity of PAs depend on their chemical structure, allowing for potency ranking and grouping. Organic cation transporter-1 (OCT1) was previously shown to be involved in the cellular uptake of the cyclic PA diesters monocrotaline, retrorsine and senescionine. However, little is known about the structure-dependent transport of PAs. Therefore, we investigated the impact of OCT1 on the uptake and toxicity of three structurally diverse PAs (heliotrine, lasiocarpine and riddelliine) differing in their degree and type of esterification in metabolically competent human liver cell models and hamster fibroblasts. Human HepG2-CYP3A4 liver cells were exposed to the respective PA in the presence or absence of the OCT1-inhibitors D-THP and quinidine, revealing a strongly attenuated cytotoxicity upon OCT1 inhibition. The same experiments were repeated in V79-CYP3A4 hamster fibroblasts, confirming that OCT1 inhibition prevents the cytotoxic effects of all tested PAs. Interestingly, OCT1 protein levels were much lower in V79-CYP3A4 than in HepG2-CYP3A4 cells, which correlated with their lower susceptibility to PA-induced cytotoxicity. The cytoprotective effect of OCT1 inhibiton was also demonstrated in primary human hepatocytes following PA exposure. Our experiments further showed that the genotoxic effects triggered by the three PAs are blocked by OCT1 inhibition as evidenced by strongly reduced γH2AX and p53 levels. Consistently, inhibition of OCT1-mediated uptake suppressed the activation of the DNA damage response (DDR) as revealed by decreased phosphorylation of checkpoint kinases upon PA treatment. In conclusion, we demonstrated that PAs, independent of their degree of esterification, are substrates for OCT1-mediated uptake into human liver cells. We further provided evidence that OCT1 inhibition prevents PA-triggered genotoxicity, DDR activation and subsequent cytotoxicity. These findings highlight the crucial role of OCT1 together with CYP3A4-dependent metabolic activation for PA toxicity.

PBTK modeling of the pyrrolizidine alkaloid retrorsine to predict liver toxicity in mouse and rat

Retrorsine is a hepatotoxic pyrrolizidine alkaloid (PA) found in herbal supplements and medicines, food and livestock feed. Dose-response studies enabling the derivation of a point of departure including a benchmark dose for risk assessment of retrorsine in humans and animals are not available. Addressing this need, a physiologically based toxicokinetic (PBTK) model of retrorsine was developed for mouse and rat. Comprehensive characterization of retrorsine toxicokinetics revealed: both the fraction absorbed from the intestine (78%) and the fraction unbound in plasma (60%) are high, hepatic membrane permeation is dominated by active uptake and not by passive diffusion, liver metabolic clearance is 4-fold higher in rat compared to mouse and renal excretion contributes to 20% of the total clearance. The PBTK model was calibrated with kinetic data from available mouse and rat studies using maximum likelihood estimation. PBTK model evaluation showed convincing goodness-of-fit for hepatic retrorsine and retrorsine-derived DNA adducts. Furthermore, the developed model allowed to translate in vitro liver toxicity data of retrorsine to in vivo dose-response data. Resulting benchmark dose confidence intervals (mg/kg bodyweight) are 24.1-88.5 in mice and 79.9-104 in rats for acute liver toxicity after oral retrorsine intake. As the PBTK model was built to enable extrapolation to different species and other PA congeners, this integrative framework constitutes a flexible tool to address gaps in the risk assessment of PA.

Structure-Dependent Toxicokinetics of Selected Pyrrolizidine Alkaloids In Vitro

Phytochemicals like pyrrolizidine alkaloids (PAs) can affect the health of humans and animals. PAs can occur for example in tea, honey or herbs. Some PAs are known to be cytotoxic, genotoxic, and carcinogenic. Upon intake of high amounts, hepatotoxic and pneumotoxic effects were observed in humans. This study aims to elucidate different toxicokinetic parameters like the uptake of PAs and their metabolism with in vitro models. We examined the transport rates of differently structured PAs (monoester, open-chained diester, cyclic diester) over a model of the intestinal barrier. After passing the intestinal barrier, PAs reach the liver, where they are metabolized into partially instable electrophilic metabolites interacting with nucleophilic centers. We investigated this process by the usage of human liver, intestinal, and lung microsomal preparations for incubation with different PAs. These results are completed with the detection of apoptosis as indicator for bioactivation of the PAs. Our results show a structure-dependent passage of PAs over the intestinal barrier. PAs are structure-dependently metabolized by liver microsomes and, to a smaller extent, by lung microsomes. The detection of apoptosis of A549 cells treated with lasiocarpine and monocrotaline following bioactivation by human liver or lung microsomes underlines this result. Conclusively, our results help to shape the picture of PA toxicokinetics which could further improve the knowledge of molecular processes leading to observed effects of PAs in vivo.

Quantitative Removal of Pyrrolizidine Alkaloids from Essential Oils by the Hydrodistillation Step in Their Manufacturing Process

Pyrrolizidine alkaloids are naturally occurring toxins produced by certain weeds that can, if accidentally co-harvested, contaminate plant-based food, feed, and herbal medicinal products. Focusing on herbal medicinal products, the presence of pyrrolizidine alkaloids is restricted by regulatory prescribed thresholds to assure patient safety. Among the multitude of different herbal active substances utilized in herbal medicinal products, the class of pharmaceutically effective essential oils is considered to exhibit a negligible contribution to pyrrolizidine alkaloid contamination. Within the present investigation, this hypothesis should be scientifically scrutinized. For this purpose, an experimental set-up was chosen that reproduces the typical manufacturing step of hydrodistillation. Essential oils of eucalyptus and lemon were selected exemplarily and spiked with 3 representative pyrrolizidine alkaloids (retrorsine, retrorsine-N-oxide, and lycopsamine), whereupon hydrodistillation was performed. Analysis of the resulting distillates by LC-MS/MS proved that artificially added pyrrolizidine alkaloids were removed completely. Moreover, quantitative pyrrolizidine alkaloid recovery in the aqueous phases was observed. Hence, it was experimentally confirmed that herbal medicinal products employing hydrodistilled essential oils of pharmaceutical quality are intrinsically free of pyrrolizidine alkaloids due to the particularities of their manufacturing process. Furthermore, it can be concluded from theoretical considerations that essential oils produced by cold pressing have a negligible risk of carrying pyrrolizidine alkaloid contamination. Our findings provide a strong indication that the requirement for analytical pyrrolizidine alkaloid testing of essential oils for pharmaceutical use should be fundamentally reconsidered.

The chemical structure impairs the intensity of genotoxic effects promoted by 1,2-unsaturated pyrrolizidine alkaloids in vitro

1,2-unsaturated pyrrolizidine alkaloids (PAs) represent a large group of secondary plant metabolites exhibiting hepatotoxic, genotoxic, and carcinogenic properties upon bioactivation. To examine how the degree of esterification affects the genotoxic profile of PA we investigated cytotoxicity, histone H2AX phosphorylation, DNA strand break induction, cell cycle perturbation, micronuclei formation, and aneugenic effects in different cell models. Analysis of cytotoxicity and phosphorylation of histone H2AX was structure- and concentration-dependent: diester-type PAs (except monocrotaline) showed more pronounced effects than monoester-type PAs. Cell cycle analysis identified that diester-type PAs induced a S-phase arrest and a decrease in the occurrence of cells in the G1-phase. The same structure-dependency was observed by flow-cytometric analysis of PA-induced micronuclei in CYP3A4-overexpressing V79 cells. Analysis of centromeres induced by lasiocarpine in the micronuclei by fluorescence in situ hybridization indicated an aneugenic effect in V79_h3A4 cells. Comet assays revealed no significant induction of DNA strand breaks for all investigated PAs. Overall, diester-type PAs induced more pronounced effects than monoester-type PAs. Furthermore, our results indicate aneugenic effects upon exposure towards lasiocarpine in vitro. These data improve our understanding how structural features of PA influence the genotoxic profile. Especially, the monoester-type PAs seem to induce less severe effects than other PAs.

Dihydroartemisinin improves hypercholesterolemia in ovariectomized mice via enhancing vectorial transport of cholesterol and bile acids from blood to bile

The increase of concentrations of total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C) in the serum of postmenopausal women is the important risk factor of the high morbidity of cardiovascular diseases of old women worldwide. To test the anti-hypercholesterolemia function of dihydroartemisinin (DHA) in postmenopausal women, ovariectomized (OVX) mice were generated, and DHA were administrated to OVX mice for 4 weeks. The blood and liver tissues were collected for biochemical and histological tests respectively. The mRNA and protein expression levels of genes related to metabolism and transport of cholesterol, bile acid and fatty acid in the liver or ileum were checked through qPCR and western blot. DHA could significantly reduce the high concentrations of TC and LDL-C in the serum and the lipid accumulation in the liver of ovariectomized mice. The expression of ABCG5/8 was reduced in liver of OVX mice, and DHA could up-regulate the expression of them. Genes of transport proteins for bile salt transport from blood to bile, including Slc10a1, Slco1b2 and Abcb11, were also significantly up-regulated by DHA. DHA also down-regulated the expression of Slc10a2 in the ileum of OVX mice to reduce the absorption of bile salts. Genes required for fatty acid synthesis and uptake, such as Fasn and CD36, were reduced in the liver of OVX mice, and DHA administration could significantly up-regulate the expression of them. These results demonstrated that DHA could improve hypercholesterolemia in OVX mice through enhancing the vectorial transport of cholesterol and bile acid from blood to bile.

Organic Cation Transporter I and Na+ /taurocholate Co-Transporting Polypeptide are Involved in Retrorsine- and Senecionine-Induced Hepatotoxicity in HepaRG cells

SCOPE

1,2-unsaturated pyrrolizidine alkaloids (PAs) are secondary plant metabolites that are found in many plant species throughout the world. They are of concern for risk assessment as consumption of contaminated foodstuff can cause severe liver damage. Of late, transporter-mediated uptake and transport has advanced as a vital determinant of PA toxicity. In this study, the authors investigate a transporter-mediated uptake of PAs and its implications in PA toxicity.

METHODS AND RESULTS

We show that transporter expression levels are significantly affected by treatment with the PAs senecionine (Sc) and retrorsine (Re) in the human hepatoma cell line HepaRG. Furthermore, the specific contribution to PA uptake of the two transporters Na⁺ /taurocholate co-transporting polypeptide (SLC10A1) and organic cation transporter I (SLC22A1), both belonging to the heterogeneous solute carrier super family, is investigated by means of a siRNA-mediated knockdown approach. Knockdown of both uptake transporters result in reduced uptake of Re and Sc in a time-dependent manner and attenuated PA-mediated cytotoxic effects in HepaRG cells.

CONCLUSION

Our results confirm previous findings of active transport mechanisms of PAs into hepatocytes and highlight the importance of toxicokinetic studies for the risk assessment of PAs.