Mistletoe compound enters clinical trials.
A drug has entered clinical trials that contains a recombinant version of the main extract from mistletoe, adn holds promise for cancer patients.
Adjuvants, Immunologic↗
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Mistletoe lectin I is a sialic acid-specific lectin with strict preference to gangliosides and glycoproteins with terminal Neu5Ac alpha 2-6Gal beta 1-4GlcNAc residues.
Mistletoe lectin I (ML-I) is a type II ribosome-inactivating protein, which inhibits the protein biosynthesis at the ribosomal level. ML-I is composed of a catalytically active A-chain with rRNA N-glycosidase activity and a B-chain with carbohydrate binding specificities. Using comparative solid-phase binding assays along with electrospray ionization tandem mass spectrometry, ML-I was shown to preferentially bind to terminally alpha2-6-sialylated neolacto series gangliosides from human granulocytes. IV(6)Neu5Ac-nLc4Cer, VI(6)Neu5Ac-nLc6Cer, and VIII(6)Neu5Ac-nLc8Cer were identified as ML-I receptors, whereas the isomeric alpha2-3-sialylated neolacto series gangliosides were not recognized. Only marginal binding of ML-I to terminal galactose residues of neutral glycosphingolipids with a Galbeta1-4Glc or Galbeta1-4GlcNAc sequence was determined, whereas a distal Galalpha1-4Gal, GalNAcbeta1-3Gal, or GalNAcbeta1-4Gal disaccharide did not bind at all. Among the glycoproteins investigated in Western blot and microwell adsorption assays, only those carrying Neu5Acalpha2-6Galbeta1-4GlcNAc residues, exclusively, predominantly, or even as less abundant constituents in an assembly with Neu5Acalpha2-3Galbeta1-4GlcNAc-terminated glycans, displayed high ML-I binding capacity. From our data we conclude that (i) ML-I has to be considered as a sialic acid- and not a galactose-specific lectin and (ii) neolacto series gangliosides and sialoglycoproteins with type II glycans, which share the Neu5Acalpha2-6Galbeta1-4GlcNAc terminus, are true ML-I receptors. This strict preference might help to explain the immunostimulatory potential of ML-I toward certain leukocyte subpopulations and its therapeutic success as a cytotoxic anticancer drug.
Adsorption↗
The discovery of three genetic races of the dwarf mistletoe Arceuthobium americanum (Viscaceae) provides insight into the evolution of parasitic angiosperms.
A population genetic approach was used to explore the evolutionary biology of the parasitic angiosperm Arceuthobium americanum Nutt. ex Engelm. (Viscaceae). Arceuthobium americanum infects three principal hosts and has the most extensive geographical range of any North American dwarf mistletoe. Based on the lack of apparent morphological and phenological differences between populations of A. americanum, past researchers have found no evidence for recognizing infraspecific taxa. In this study, molecular analysis using amplified fragment length polymorphism (AFLP) analysis indicated that A. americanum is divided into three distinct genetic races, each associated with a different host taxon in regions of allopatry: (i) Pinus banksiana in western Canada; (ii) Pinus contorta var. murrayana in the Sierra Nevada and Cascade Mountain ranges in the western US; and (iii) Pinus contorta var. latifolia in the western US and Canada. These observations suggest that host identity, geographical isolation and environmental factors have contributed to race formation in A. americanum. The lack of fine-scale patterning within each of the A. americanum races is attributed to random dispersal of seeds over long distances by animal vectors. Historical factors such as glaciations and founder events have also influenced structuring and genetic diversity in A. americanum populations. Given sufficient time, it is possible that these races will become reproductively isolated and undergo speciation.
Biological Evolution↗
Cytostatic and cytocidal effects of mistletoe (Viscum album L.) quercus extract Iscador.
Mistletoe (Viscum album) extract (Iscador) is used either alone or in combination with chemotherapy or radiotherapy in the treatment of tumor patients. In this study the effects of Viscum album Quercus (VA Qu) extract (1000 ng lectin and 6 microg viscotoxin/ml) in various doses were investigated in an in vitro model with tumor cells: three multiple myeloma (MM) cell lines (OPM-2, RPMI-8226, U-266) and three B cell lymphoma cell lines (U-698, DOHH-2, WSU-1). None of the three B lymphoma cell lines and none of the three multiple myeloma cell lines produced interleukin (IL)-6 spontaneously or after treatment with VA Qu extract. All three multiple myeloma cell lines expressed surface IL-6R and gp130, the B cell lymphomas expressed only gp130. Viscum album/Qu extract markedly downregulated the membrane expression of IL-6R and gp130 in RPMI-8226 (down to 29% and 32%) and the expression of gp130 in WSU-1 (down to 22%). There was a marked reduction of viable cells of RPMI-8226 (down to 28%) and WSU-1 (down to 8 %) at 100 microg/10(6) cells /ml. There was a clear relationship between the inhibition of proliferation and viability: the percentage reductions of the viable cells at 48 and 72 h were similar to those of proliferation at 24 and 48 h, respectively. This means that firstly the proliferation of the tumor cell is inhibited and then afterwards these cells die by apoptosis or necrosis. The inhibitory effect of VA Qu on the proliferation can be termed cytostatic, on the survival cytocidal. VA Qu was more effective in cells having a high proliferation rate than in those with a low proliferation rate. The effective dose range lay between 25 and 100 microg/10(6) cells/ml (5-20 ng lectin/10(6) cells/ml) for all parameters.
Antineoplastic Agents, Phytogenic↗
Effects of mistletoe (Viscum album L.) extracts Iscador on cell cycle and survival of tumor cells.
The molecular and cellular mechanisms by which mistletoe (Viscum album L.) extracts exert cytotoxic and immunomodulatory anti-tumoral effects are largely unknown. In this study the hypothesis that Iscador preparations induce tumor regression by cell cycle inhibition and/or interference with apoptotic signaling pathways in cancer cells was investigated. Also a possible effect on angiogenesis, which is a prerequisite for tumor growth in vivo, is studied in endothelial cell cultures. Furthermore, it was examined which apoptotic signaling route(s) is (are) activated by Iscador by studying specific pro-apoptotic proteins in cultured cells. To characterize these properties, 9 human cancer cell lines of different origin, one epidermis derived cell line and 2 endothelial cell cultures were incubated with different concentrations of Iscador Quercus Spezial and Iscador Malus Spezial. Cell cycle kinetic parameters were measured by bromodeoxyuridine (BrdU) pulse labeling and tubulin staining. Apoptotic responses were detected by M30 Cyto-Death or Annexin V/propidium iodide assays. Characterization of the apoptotic pathway(s) was performed by staining cells for amongst others active caspase 3 and cytochrome C (mitochondrial pathway), as well as active caspase 8 (death receptor pathway). The sensitivity to Iscador treatment varies strongly between different cell lines and also ing those derived from small cell lung cancer, and adenocarcinoma of the lung and breast, as well as endothelial cell cultures, Iscador caused early cell cycle inhibition followed by apoptosis in a dose dependent manner. Amongst the low responders are cell lines derived from colorectal carcinoma. In general Iscador Malus exerted a stronger response than Iscador Quercus. Apoptosis was induced by activating the mitochondrial but not the death receptor dependent pathway, at least in case of Iscador Quercus. Iscador Malus also seemed to induce apoptosis via the death receptor route, which may explain the higher sensitivity of cancer and endothelial cells to this preparation.
Annexin A5↗
Preclinical investigations with mistletoe (Viscum album L.) extract Iscador.
The mistletoe (Viscum album L.) extract Iscador has been shown to be an effective complementary drug in the treatment of cancer patients after surgical removal of the primary tumor. It improved survival, recovery from damage caused by irradiation or cytostatic therapy, and quality of life. Beneficial effects were seen especially as reduction of the side effects caused by basic oncological therapy. In animal tests, clear anti-carcinogenic effects of Iscador were demonstrated as reduction of tumor growth in preformed tumors and as prevention of tumor growth in newly induced tumors. Mainly immune stimulation but also direct cytotoxic activity are believed to be responsible for the anti-carcinogenic activity of Iscador. Other effects seen in patients, such as improved quality of life and improved psychic conditions, are not possible to be tested in animals. Recently, toxicological investigations were performed with Iscador to get more information on possible toxicological effects which can only be evaluated in preclinical studies. Tests examining the acute toxicity, genotoxic effects as well as effects on reproduction were performed. In these studies, no adverse effects of Iscador preparations were detected thus confirming the information on the safety of the extract which has been gained in clinical trials and during more than 80 years of use in human therapy. Iscador has been shown to be essentially safe. No severe adverse events have been reported during many years of use by thousands of patients. Genotoxic effects and effects on reproduction, which cannot be evaluated in clinical use, have been cleared up in animal tests. Iscador was shown to be clearly non-genotoxic and free of relevant toxic effects on reproduction in vivo. In summary, no risk of adverse effects of Iscador during human use is expected.
Animals↗
Molecular genetic survey of European mistletoe (Viscum album) subspecies with allele-specific and dCAPS type markers specific for chloroplast and nuclear DNA sequences.
The qualitative and quantitative content of mistletoe metabolites, and bioactivity of extracts is related to the subspecies of Viscum album L. These were indicated to be genetically distinct and host specific. We aimed to check (i) whether the specificity is strict and (ii) how frequently hybridization occurs among the subspecies. We designed two sets of allele-specific and dCAPS molecular genetic markers that would facilitate identification of Viscum album L. subspecies and their hybrid derivatives on the basis of chloroplast trnH(GUG)- trnK(UUU) and nuclear rDNA ITS1&2 sequences. Out of 118 plants surveyed, 103 displayed characteristics that confirmed strict host specificity of the subspecies, in addition, the results were compliant between nuclear and chloroplast markers showing no indication of hybridization among subspecies. From 15 samples that showed deviations from this model 13 came from the Mediterranean Sea basin, and only two originated from Central and Western Europe. Abbreviations. dCAPS:derived Cleaved Amplified Polymorphic Sequence ITS1&2:Internal Transcribed Spacers 1&2 MAMA:Mismatch Amplification Mutation Assay
Alleles↗
Crystal structure of himalayan mistletoe ribosome-inactivating protein reveals the presence of a natural inhibitor and a new functionally active sugar-binding site.
Ribosome-inactivating proteins (RIPs) are toxins involved in plant defense. How the plant prevents autotoxicity is not yet fully understood. The present study is the first structural evidence of a naturally inhibited form of RIP from a plant. Himalayan mistletoe RIP (HmRIP) was purified from Viscum album leaves and crystallized with lactose. The structure was determined by the molecular replacement method and refined at 2.8-A resolution. The crystal structure revealed the presence of high quality non-protein electron density at the active site, into which a pteridine derivative (2-amino 4-isopropyl 6-carboxyl pteridine) was modeled. The carboxyl group of the ligand binds strongly with the key active site residue Arg(162), nullifies the positive charge required for catalysis, and thereby acts as a natural inhibitor. Lectin subunits of RIPs have two active sugar-binding sites present in 1alpha- and 2gamma-subdomains. A third functionally active site has been identified in the 1beta-subdomain of HmRIP. The 1beta-site is active despite the absence of conserved polar sugar-binding residues. Loss of these residues is compensated by the following: (i) the presence of an extended site where the penultimate sugar also interacts with the protein; (ii) the interactions of galactose with the protein main chain carbonyl and amide nitrogen atoms; (iii) the presence of a well defined pocket encircled by four walls; and (iv) a favorable stacking of the galactose ring with Tyr(66) besides the conserved Phe(75). The mode of sugar binding is also distinct at the 1alpha and 2gamma sugar-binding sites.
Arginine↗
Evidence for stimulation of tumor proliferation in cell lines and histotypic cultures by clinically relevant low doses of the galactoside-binding mistletoe lectin, a component of proprietary extracts.
The toxic galactoside-specific lectin from mistletoe, a component of proprietary extracts with unproven efficacy in oncology, exhibits capacity to trigger enhanced secretion of proinflammatory cytokines at low doses (ng/ml or ng/kg body weight) and reductions of cell viability with increasing concentrations. To infer any tumor selectivity of this activity, cytofluorimetric and cell growth assays with a variety of established human tumor cell lines were performed. Only quantitative changes were apparent, and the toxicity against tumor cells was within the range of that of the tested fibroblast preparations from 5 donors. No indication for any tumor selectivity was observed. In kinetic studies with 8 sarcoma and 4 melanoma lines, this evidence for quantitative variability of the response in interindividual comparison was further underscored. At 50 pg lectin/ml x 10(5) cells, even a growth-stimulatory impact was noted in 5 of 12 tested cases. To mimic in vivo conditions with presence of cytokine-secreting inflammatory and stromal cells, exposure to the lectin was extended to histotypic cultures established from 30 cases of surgically removed tumor. As salient result, 5 specimens from 4 of the 8 tested tumor classes responded with a significant increase of [3H]-thymidine incorporation relative to controls during the culture period of 72 hours, when the lectin was present at a concentration in the described immunomodulatory range (1 ng/ml). A relation of this activity to the extent of the actual proliferative status of the reactive samples could not be delineated. Therefore, a non-negligible percentage of the established tumor cell lines (e.g., 3 from 8 sarcoma lines) can be markedly stimulated by the lectin at a very low dose and with dependence on the cell type. Furthermore, the feasibility to elicit a significant growth enhancement is likewise documented for human tumor explants in 16.6% of the examined cases. In view of the uncontrolled application of lectin-containing extracts in alternative/complementary medicine, the presented results on unquestionably adverse lectin-dependent effects in two culture systems call for rigorous examination of the clinical safety of this unconventional, scientifically entirely experimental treatment modality.
Biotinylation↗
Preferential binding of the anticancer drug rViscumin (recombinant mistletoe lectin) to terminally alpha2-6-sialylated neolacto-series gangliosides.
Production of biochemically defined recombinant mistletoe lectin was achieved by cloning and separate expression of the single catalytically active A-chain and the B-chain with carbohydrate binding properties in Escherichia coli, yielding an active heterodimeric protein named rViscumin (Eck et al. [1999] Eur. J. Biochem., 265, 788-797). Employing solid phase binding assays, rViscumin was shown to preferentially bind to terminally alpha2-6-sialylated neolacto-series gangliosides IV(6)Neu5Ac-nLc4Cer, VI(6)Neu5Ac-nLc6Cer, and VIII(6)Neu5Ac-nLc8Cer isolated from human granulocytes. Only marginal binding of rViscumin to galactose-terminated neutral GSLs was determined, whereas reinvestigation of ricin specificity demonstrated this lectin as a galactose-binding protein. Human promyelotic HL-60 cells exhibited an IC(50) value (half maximum cytotoxicity) of 1.16 pM and human bladder carcinoma 5637 cells of 12.1 pM rViscumin; CHO-K1 cells were resistant to rViscumin treatment up to a concentration of 5.26 nM tested. Quantification of the predominant receptor ganglioside IV(6)Neu5Ac-nLc4Cer by means of a specific anti-Neu5Acalpha2-6Galbeta1-4GlcNAc-R antibody revealed 3.68 x 10(6) and 1.54 x 10(6) receptor molecules per HL-60 and 5637 cell, respectively; CHO-K1 cells were negative, lacking alpha2-6-sialylated gangliosides. The data imply a direct correlation of rViscumin cytotoxicity and the expression of receptor ganglioside. Moreover, CHO-K1 cells were rendered susceptible toward rViscumin cytotoxicity after exogenous application of human granulocyte gangliosides. Thus, (1) rViscumin has to be considered as a sialic acid-specific rather than a galactose-specific type II ribosome-inactivating protein, and (2) neolacto-series gangliosides with Neu5Acalpha2-6Galbeta1-4GlcNAc-terminus are true functional and physiologically relevant rViscumin receptors.
Animals↗
Crystallisation under microgravity of mistletoe lectin I from Viscum album with adenine monophosphate and the crystal structure at 1.9 A resolution.
The crystal structure of the ribosome-inactivating protein (RIP) mistletoe lectin I (ML-I) from Viscum album in complex with adenine has been refined to 1.9 A resolution. High quality crystals of the ML-I complex were obtained by the method of vapour diffusion using the high density protein crystal growth system (HDPCG) on the international space station, mission ISS 6A. Hexagonal crystals were grown during three months under microgravity conditions. Diffraction data to 1.9A were collected applying synchrotron radiation and cryo- techniques. The structure was refined subsequently to analyse the structure of ML-I and particularly the active site conformation, complexed by adenine that mimics the RNA substrate binding.
Adenosine Monophosphate↗
Mistletoe viscotoxins induce membrane permeabilization and spore death in phytopathogenic fungi.
Viscotoxins (Vts) are basic peptides expressed in mistletoe leaves, seeds and stems which have been shown to be cytotoxic to mammalian cells. The aim of this study was to analyse whether Vts were able to control and/or inhibit the growth of phytopathogenic fungi to obtain a clue to their biological function. Incubation of two Vt isoforms, VtA(3) and VtB, at a final concentration of 10 micro M resulted in a complete blockage of the germination of spores from three different pathogenic fungi. It was also shown that lower concentrations than 10 micro M of VtA(3) and VtB inhibit their mycelial growth in a dose-dependent manner. The protein dose required to inhibit the growth of Fusarium solani and Sclerotinia sclerotiorum to a 50% was between 1.5 and 3.75 micro M, which represents a potent activity. No significant differences in the antifungal potency for each Vt isoform, either VtA(3) and VtB, were observed, although they have been shown to exert differential cytotoxicity on mammalian cells. It was also demonstrated that Vts act as fungicidal compounds. To explore the basis of the antifungal activity the ability of VtA(3) to induce changes in membrane permeability and on the oxidative status of F. solani spores was analysed. By using a specific fluorescent probe on intact spores, it was demonstrated that VtA(3) produces rapid changes in fungal membrane permeability. It also induces H(2)O(2) production verified by a histochemical staining. The data presented in this study support a direct role of Vts in the plant defence determined by their lethal effect on fungal pathogens.
Journal Article↗
Critical role of reactive oxygen species and mitochondrial membrane potential in Korean mistletoe lectin-induced apoptosis in human hepatocarcinoma cells.
Viscum album L. coloratum agglutinin (VCA), isolated from Korean mistletoe, is a strong inducer of apoptosis in a variety of tumor cells; however, the underlying molecular mechanisms responsible are not clear. Here, we show that VCA induces apoptotic killing, as demonstrated by DNA fragmentation, Hoechst 33258 staining, terminal deoxynucleotidyl transferase dUTP nick-end labeling assay, and flow cytometry analysis in hepatocarcinoma Hep3B cells. VCA treatment results in a significant increase in reactive oxygen species (ROS) and loss of mitochondrial membrane potential (DeltaPsim). Furthermore, treatment with the antioxidant N-acetyl-L-cysteine reduces ROS induction by VCA, preventing apoptosis in Hep3B cells, indicating that oxidative stress is involved in VCA-mediated cell death. Our results also show rapid changes in mitochondrial transition permeability, Bax translocation, cytochrome c release, caspase-3 activity, and poly(ADP-ribose) polymerase degradation in Hep3B cells occurring in VCA-induced apoptosis. There is much evidence that implicates c-Jun NH2-terminal kinase (JNK) activation with apoptosis in a variety of cellular and animal models. In this study, we show that VCA induces JNK phosphorylation, which is abolished with pretreatment with a JNK inhibitor. Moreover, Hep3B cells overexpressing JNK1 or stress-activated protein kinase kinase (SEK1) seem to be more susceptible to cell death from ROS and loss of DeltaPsim induced by VCA, whereas expression of dominant-negative JNK1 or SEK1 in Hep3B cells do not. These data suggest that JNK phosphorylation may be a major regulator involved in VCA-induced apoptosis. Together, these results suggest that VCA induces apoptosis by inducing ROS production and a loss of DeltaPsim, in which JNK phosphorylation plays a critical role in these events.
Apoptosis↗
Mistletoe and gemcitabine in patients with advanced cancer: a model for the phase I study of botanicals and botanical-drug interactions in cancer therapy.
Plant extracts of the European mistletoe (MTE), Viscum album, the most widely used cancer treatment in Germany, have been used in European countries as sole intervention or as adjunct to conventional cancer therapies for more than 80 years. Preclinical data suggest immunostimulatory and cytotoxic effects of MTE. While the clinical efficacy of MTE in cancer is being investigated, toxicity and potential interactions of MTE with standard chemotherapeutic agents are unknown. Gemcitabine is an approved antimetabolite chemotherapeutic agent effective as single agent in patients with solid tumors (ST). The documented metabolism and pharmacokinetics of gemcitabine make this agent well suited for the study of botanical-chemotherapy drug interactions (BDIA) in cancer. Based on reports of altered drug metabolism associated with botanical preparations, research into BDIA has intensified. The phase I, 2-stage, dose-escalation study outlined here will test MTE with gemcitabine as a paradigm for the phase I investigation of botanical-drug combination treatments in patients with advanced ST. The protocol including the following components has been reviewed and approved by the National Cancer Institute Institutional Review Board (IRB), the National Naval Medical Center IRB, and the Navy Clinical Investigation Program (study 02-074): (1) use of a standardized MTE, approved by the Food and Drug Administration for investigational use; (2) independent verification of key MTE components considered biologically active; (3) identification of contaminants and adulterants; (4) pharmacokinetics of gemcitabine and its principal metabolites before and upon exposure to MTE; (5) safety and toxicity data collection; (6) assays of plasma ML antibody production in vivo; and (7) pharmacodynamic studies of the botanical-drug combination.
Antineoplastic Combined Chemotherapy Protocols↗
Studies on the constituents of Japanese mistletoes from different host trees, and their antimicrobial and hypotensive properties.
The chemical constituents of Japanese mistletoes, Taxillus yadoriki Danser, Taxillus kaempferi Danser, and Korthalsella japonica Engler, epiphyting to different host trees were compared, and the antimicrobial and hypotensive properties of some isolated flavonoids were examined. Two known flavonoid glycosides, hyperin and quercitrin, were isolated from Taxillus yadoriki Danser, together with fatty acids, phytosterol, and phytosterol-glucoside. There was remarkable variation of contents of quercitrin among the plants on different host trees. From Taxillus kaempferi Danser, fatty acids, phytosterol, phytosterol-glucoside, quercetin, avicularin, and taxillusin were isolated, and quercitrin and hyperin were also identified. There was no remarkable variation of compositions of flavonoid glycosides among the plants on different host trees. A known flavone glycoside, chrysoeriol-4'-O-glucoside, was isolated from Korthalsella japonica Engler, together with fatty acids, phytosterol, oleanolic acid, and phytosterol-glucoside. Chrysoeriol-4'-O-glucoside is contained in this plant irrespective of the host trees.
Animals↗
Mistletoe lectin I-induced effects on human cytotoxic lymphocytes. I. Synergism with IL-2 in the induction of enhanced LAK cytotoxicity.
This report demonstrates that in vitro activation of human cells with the beta-galactoside-specific lectin from mistletoe (ML-I) or interleukin-2 (IL-2) results in different patterns of activation and function of cytotoxic cells. It is now well established that natural killer (NK) and lymphokine-activated killer (LAK) cytotoxicity is mainly mediated by resting (NK) and IL-2-activated (LAK) CD56-positive (+) cells respectively. Culture of peripheral blood lymphocytes (PBL) for 3 days with ML-I led to expansion and activation of T cells which demonstrated NK- and LAK-like cytotoxicity. T lymphocyte subset analysis revealed that in total PBL, ML-I preferentially stimulated and expanded CD8+ T cells which mediated the cytotoxic effect. Incubation of highly purified CD8+ T cells alone with ML-I did not lead to induction of cytotoxicity, which required the presence of both CD4+ and CD14+ (monocytes) cells, suggesting that ML-I does not exert a direct effect on CD8+ T cells. Activation of PBL with both ML-I and IL-2 resulted in simultaneous induction of T and CD56+ cell-mediated NK and LAK cytotoxicity. These data suggest that treatment with ML-I and IL-2 might provide an approach to induce maximum cytotoxicity against tumors and to recruit both T and NK cells for tumor therapy.
CD4-Positive T-Lymphocytes↗
Mistletoe lectin activates caspase-8/FLICE independently of death receptor signaling and enhances anticancer drug-induced apoptosis.
Mistletoe lectin I (ML-I) is a major active component in plant extracts of Viscum album that is increasingly used in adjuvant cancer therapy. ML-I exerts potent immunomodulating and cytotoxic effects, although its mechanism of action is largely unknown. We show that treatment of leukemic T- and B-cell lines with ML-I induced apoptosis, which required the prior activation of proteases of the caspase family. The involvement of caspases is demonstrated because (a) a peptide caspase inhibitor almost completely prevented ML-I-induced cell death and (b) proteolytic activation of caspase-8, caspase-9, and caspase-3 was observed. Because caspase-8 has been implicated as a regulator of apoptosis mediated by death receptors, we further investigated a potential receptor involvement in ML-I-induced effects. Cell death triggered by ML-I was neither attenuated in cell clones resistant to CD95 nor in cells that were rendered refractory to other death receptors by overexpressing a dominant-negative FADD mutant. In contrast, ML-I triggered a receptor-independent mitochondria-controlled apoptotic pathway because it rapidly induced the release of cytochrome c into the cytosol. Because ML-I was also observed to enhance the cytotoxic effect of chemotherapeutic drugs, these data may provide a molecular basis for clinical trials using MLs in anticancer therapy.
Amino Acid Chloromethyl Ketones↗
Induction of Fas ligand (CD95L) by the toxic mistletoe lectins in human lymphocytes.
BACKGROUND: Fas ligand (FasL, CD95L) predominantly expressed on activated cytotoxic T cells and NK cells triggers apoptosis in Fas receptor (Apo-1, CD95) positive target cells. We investigated the expression of FasL, Fas and tumor necrosis factor (TNF) receptor 1 (TNF-R1, CD120a) on cultured human lymphocytes and leukemic T and B cells. MATERIALS AND METHODS: Lymphocytes from six healthy individuals, from four patients with chronic lymphocytic T or B cell leukaemia, and leukemic Molt-4 cells were incubated with the apoptosis- inducing mistletoe lectins (ML I and ML III). RESULTS: Incubation of differentiated lymphocytes with the ML resulted in a significant upregulation of FasL in the surviving CD4+ T helper cells, CD8+ cells and CD19+ B cells. Similarly, the TNF receptor expression increased, while the Fas molecule decreased. In contrast, FasL was not induced in leukemic cells. CONCLUSIONS: Apart from a direct induction of apoptosis in response to an inhibition of protein synthesis by the enzymic ML A chain, ML treatment may indirectly induce apoptosis in Fas+ tumour cells through activated FasL+ lymphocytes.
Antigens, CD↗