Activation of artemisinin and heme degradation in Leishmania tarentolae promastigotes: A possible link.

Endoperoxides (EPs) appear to be promising drug candidates against protozoal diseases, including malaria and leishmaniasis. Previous studies have shown that these drugs need an intracellular activation to exert their pharmacological potential. The efficiency of these drugs is linked to the extensive iron demand of these intracellular protozoal parasites. An essential step of the activation mechanism of these drugs is the formation of radicals in Leishmania. Iron is a known trigger for intracellular radical formation. However, the activation of EPs by low molecular iron or by heme iron may strongly depend on the structure of the EPs themselves. In this study, we focused on the activation of artemisinin (Art) in Leishmania tarentolae promastigotes (LtP) in comparison to reference compounds. Viability assays in different media in the presence of different iron sources (hemin/fetal calf serum) showed that IC50 values of Art in LtP were modulated by assay conditions, but overall were within the low micromolar range. Low temperature electron paramagnetic resonance (EPR) spectroscopy of LtP showed that Art shifted the redox state of the labile iron pool less than the EP ascaridole questioning its role as a major activator of Art in LtP. Based on the high reactivity of Art with hemin in previous biomimetic experiments, we focused on putative heme-metabolizing enzymes in Leishmania, which were so far not well described. Inhibitors of mammalian heme oxygenase (HO; tin and chromium mesoporphyrin) acted antagonistically to Art in LtP and boosted its IC50 value for several magnitudes. By inductively coupled plasma methods (ICP-OES, ICP-MS) we showed that these inhibitors do not block iron (heme) accumulation, but are taken up and act within LtP. These inhibitors blocked the conversion of hemin to bilirubin in LtP homogenates, suggesting that an HO-like enzyme activity in LtP exists. NADPH-dependent degradation of Art and hemin was highest in the small granule and microsomal fractions of LtP. Photometric measurements in the model Art/hemin demonstrated that hemin requires reduction to heme and that subsequently an Art/heme complex (λmax 474 nm) is formed. EPR spin-trapping in the system Art/hemin revealed that NADPH, ascorbate and cysteine are suitable reductants and finally activate Art to acyl-carbon centered radicals. These findings suggest that heme is a major activator of Art in LtP either via HO-like enzyme activities and/or chemical interaction of heme with Art.

Techniques to study phagocytosis and uptake of Leishmania tarentolae by J774 macrophages.

Leishmaniasis is a vector borne parasitic disease affecting millions of people worldwide and is spreading into further areas because of global warming. The development of new active substances against these single-cell eukaryotic parasites is of great importance. Leishmania tarentolae promastigotes (LtP) are non-pathogenic for mammals and serve as model organisms for pathogenic Leishmania in basic research. However, it is important to refine methods to study the process of the infection of mammalian macrophages by LtP and pathogenic Leishmania. Important stages of the infection are phagocytosis by macrophages and multiplication of Leishmania amastigotes in the phagolysosome of macrophages. In this study, advanced methods using electron spin resonance (ESR) spectroscopy and genetically manipulated LtP were used to monitor the infection of adherent J774 macrophages with LtP. An ESR method was established to detect the formation of superoxide radicals directly in adherent J774 cells and to investigate the effect of LtP on this activity. J774 cells responded with a burst of superoxide radicals in the presence of phorbol myristate acetate as positive control. In contrast, challenging J774 cells with LtP resulted in a much lower burst of superoxide radicals. To facilitate LtP detection in the phagolysosome of J774 macrophages, LtP expressing enhanced green fluorescent protein (EGFP-LtP) were constructed. After different infection times with EGFP-LtP, the J774 cells were visualized by phase contrast microscopy and the cell number was determined. The intramacrophage Leishmania tarentolae amastigotes (LtA) expressing EGFP were detected by fluorescence microscopy and then counted with ImageJ. These experiments showed that LtP are taken up by J774 cells and form intraphagolysosomal amastigotes. LtA under our conditions multiplied intracellularly and were able to persist about 48 h in J774 cells. These experiments showed that ESR spectroscopy of attached macrophages and the use of the EGFP-LtP are suitable methods to study the initial phase of Leishmania infection in vitro.

Interaction of ascaridole, carvacrol, and caryophyllene oxide from essential oil of Chenopodium ambrosioides L. with mitochondria in Leishmania and other eukaryotes.

The antileishmanial activity of the essential oil (EO) from Chenopodium ambrosioides L. has been demonstrated in vitro and in animal models, attributed to the major components of the EO. This study focused on the effects of the three major EO compounds carvacrol, caryophyllene oxide (Caryo), and the antileishmanial endoperoxide ascaridole (Asc) on mitochondrial functions in Leishmania tarentolae promastigotes (LtP). EO and Caryo were able to partially inhibit the leishmanial electron transport chain, whereas other components failed to demonstrate a direct immediate effect. Caryo demonstrated inhibition of complex III activity in LtP and in isolated complex III from other species. The formation of superoxide radicals was studied in Leishmania by electron spin resonance spectroscopy in the presence of iron chelators wherein selected compounds failed to trigger a significant immediate additional superoxide production in LtP. However, upon prolonged incubation of Leishmania with Asc and especially in the absence of iron chelators (allowing the activation of Asc), an increased superoxide radical production and significant impairment of mitochondrial coupling in Leishmania was observed. Prolonged incubation with all EO components resulted in thiol depletion. Taken together, the major components of EO mediate their leishmanicidal activity via different mitochondrial targets and time profiles. Further studies are required to elucidate possible synergistic effects of carvacrol and Asc and the influence of minor compounds.

Mechanism of ascaridole activation in Leishmania.

Endoperoxides (EP) are an emerging class of drugs which have potential in antiparasitic therapy, but also in other fields. For malaria therapy the EP artemisinin (Art) and its derivatives are successfully used. We have shown in the past that the EP ascaridole (Asc) is useful for the treatment of cutaneous leishmaniasis in a mouse model. Biomimetic experiments suggested that these drugs need activation in the respective target pathogens to exert their function. In spite of this idea, direct activation of EP to radicals inside cells has never been demonstrated. Therefore, this study was initiated to explore the activation of Asc in biomimetic systems and inside Leishmania in comparison to Art. Using electron paramagnetic resonance spectroscopy (EPR) in combination with spin-trapping we identified the secondary alkyl radical intermediates arising from reduction by Fe2+ in cell-free systems. Combined GC/NMR analysis confirmed the loss of isopropyl residues from Asc during this process as intermediates. This activation of Asc was stimulated by low molecular Fe2+ complexes or alternatively by hemin in conjunction with thiol reductants, such as cysteine (Cys). In Leishmania tarentolae promastigotes (LtP) as model for pathogenic forms of Leishmania carbon-centered radicals were identified in the presence of Asc by EPR spin-trapping. Both Asc and Art inhibited the viability in LtP with IC50 values in the low micromolar range while IC50 values for J774 macrophages were considerably higher. A similar structure without EP bridge (1,4-cineole) resulted in no detectable radicals and possessed much less cytotoxicity in LtP and no selectivity for LtP compared to J774 cells. The Asc-derived radical formation in LtP was inhibited by the iron chelator deferoxamine (DFO), and stimulated by Cys (a suitable reductant for hemin). The IC50 values for LtP viability in the presence of Asc or Art were increased significantly by the spin trap DMPO, while Cys and DFO increased only IC50 values for Art. In a heme association assay Asc demonstrated a lower binding affinity to heme than Art. ICP-OES measurements revealed that in LtP the total iron concentrations were twice as high as values in J774 macrophages. Since low molecular iron was important in Asc activation we studied the influence of Asc on the labile iron pool (LIP) in LtP. Low temperature EPR experiments demonstrated that Asc shifts the redox balance of iron in the LIP to its oxidized state. These data demonstrate that univalent cleavage of Asc/Art in LtP is an essential part of their pharmacological mechanism. The structure of the EP determines whether activation by low molecular iron or heme is favored and the availability of these intracellular activators modulates their cytotoxicity. These findings may be helpful for synthesis of new Asc derivatives and understanding the action of EP in other cell types.